Thoughts and experiences on efficient pet-rearing!
As a completionist, I of course had to get a complete collection of possible pets. And then, of course, I had to level them all to 20. In so doing, I've had the opportunity to learn a few things about doing so efficiently, which I will share here, in the hopes that it will benefit others. But first, let's just sum up three fundamental facts that inform my conclusions...
Firstly, you pet ONLY gains XP by being fed. Whether it's joining you in pulse-pounding combat, or spending the rest of its time snoring on a Pet Bed, is irrelevant - only mealtimes matter!
Secondly, while I could be wrong about this - I haven't tested it that extensively - it seems like the XP your pet gains from a meal is purely linked to that meal's Hunger Fill. Meaning, Thirst Fill is irrelevant, which also means that most soups, stews and so on make poor pet-food, since they tend to be balanced in that regard.
Third, your pet will get hungry every 8 in-game hours, regardless of how filling their last meal was.
So, from this, you can draw a simple conclusion - you need food with the highest possible Hunger Fill, if you want to level them quickly! Which leads us to an unfortunate detail about the Cooking-system. As you level up Cooking, you get a rather hefty bonus to Hunger and Thirst Fill for anything you fry or bake, but ONLY for that, and this benefit is COMPLETELY non-transferable. So, anything you prepare at the Chef's Counter isn't affected by your skill, even if it uses fried or baked foods as ingredients, as many such meals do. Now portionable food - cakes, pies, casseroles, etc. - DO allow this quality to transfer to the actual portions that you eat, but unfortunately, none of those portions have high enough Hunger Fill for this to really matter.
What this means is that a LOT of cooking-options just kind of plateau - they may be a decent fallback, or useful in particular situations, but they never get better. Cosmato Salad Wrap, for instance, often gets mentioned as an excellent source of food for you OR your pet, since all the ingredients can be farmed - but it has 51.7 Hunger Fill, and that is all it will ever have. Still, it can be a good fallback, since you'll never run out! By the same token, some of the more high-end portionable foods - Simple Pest Pie, Carrot Cake, Salted Honey Pie - will, at high Cooking-levels, reach a Hunger Fill rate of around 50, making them comparable while being highly resource-effective thanks to the Serving Seconds perk, and able to stack up to 6 instead of 4 as with most food-items. This may also make them useful if you're planning a long expedition and need to bring enough food for both yourself and a pet, while fitting it all in a single cooler-space!
However, the real take-away is that for optimum leveling, you need fried or baked foods with high Hunger Fill and no portioning. There are three stand-outs on that front - the shared 1st Place goes to the Fr-eyed Rice and the Egg & Peccary Tartine, with a close second being the Stuffed Roast Peccary. All three can easily clear 100 Hunger Fill at high Cooking-levels. Of course, none of those can be made with purely home-grown ingredients, but it isn't really far off! Rice, Egg, Tomato, Mushroom, Greyeb, plus Wheat and Milk for Bread, can all be grown at home - just make sure to tame a Peccary Sow for milking, and potentially a Mushroom Peccary for Mushrooms, allowing you to grow those even before you reach Canaan and get the ability to craft an actual Mushroom Farm.
What you'll be missing is Canned Peas, Salt, and Meat. Salt is easy enough to get, by several different routes, and if you care about cooking at all, you'll be grabbing it everywhere you go regardless. If you need a lot, though, Darkwater Fish make a good, renewable source! Canned Peas, meanwhile, is something you just need to make a habit of collecting any chance you get - there's a bunch in Flathill, right near the start, for easy collection after every reset, and a couple in the Witch's Hut in Canaan which you're likely to repeatedly visit for... other reasons, anyway, so why not grab those two while you're at it? You can also trade Root Beer for Canned Peas at the Security Station in the Office Lobby, so make a point of emptying every vending-machine you come across of Root Beer - and of course, you can also grab a can every time you take down a Security or Containment Bot! Considering all of that, you can easily get a huge stockpile.
So, all that remains is to just make sure to kill and butcher every Peccary you meet, for Drumsticks to stuff and Peccary Chops that you can grind into Sausage. Or, if you really want to optimize, just stick with the Fr-eyed Rice, which needs only home-grown veggies and Canned Peas, along with the Egg & Peccary Tartine, in order to avoid having two dishes compete for a limited supply of Peas - especially since the Stuffed Peccary is slightly inferior to the other two. Just means you can leave all those Drumsticks to rot, I suppose!
Honorable mention goes to the Silky Fish Florentine - the third-place finisher. It tops out at just shy of 90 Hunger Fill, which really isn't bad, though it also comes in quite late, since the Silken Betta is one of the last fish you're likely to catch. Still, you can make two Florentines per fish, and the rest of the ingredients are home-grown, so if you caught a bunch anyway - perhaps while trying for that elusive Gossamer Betta - you can do a lot worse than turning it all into a smorgasbord of Florentines. Just keep in mind that it'll be competing with the Egg & Peccary Tartine for Milk, and with the Stuffed Roast Peccary for Mushrooms...