So the thing is, if you put more than one turnip / cucumber down it will actually combine them in a way that when you make it with just one turnip, they won't stack. I assume this is because you optionally used more ingredients to make it heal more stamina? That's sort of what I've gotten from the mechanic so far. If I make Happy Eggplant for example with Sugar in the frying pan, it heals me more but also does not stack with the normal ones. SO I guess its a pseudo quality thing.
Correct. At least for 0.5. I rewrote parts of the cooking for 0.6. So you'll get a meal for every complete set of ingredients. So two eggplants will now give you two happy eggplants. Which makes more sense... Although that is funny when you have items that require two ingredients, if you use one of x and 19 of y, you'll still get one of your recipe, but it will be a super recipe, restoring a load of hunger/sat in one go xD.