If you want to heal all your hearts no matter what is in the recipe, and get some bonus hearts, cook with hearty food. Even if it is just one item, such as a Hearty Durian, will get Full Heart Recovery + more hearts. This means you can literally save all base cooking items that just give hearts as filler for cooking, or for cooking for the soul purpose of selling it.
This means 1 Hearty Bass is better than 5 slabs of meat stacked.
And no, adding normal food to hearty food in cooking does not increase the bonus heart total. Only adding more hearty materials will. Not sure about Palm fruit, will test… Tested, no difference in bonus hearts. Same for salt.
If you want more of these items just set your Radar + to any heart food after you drop it on the ground and take a picture of it. Each picture will give a hint as to where the item can be found.
Also, it might be more worth it to also worth it to cook a stack of 5 items, sell it, and use the money to buy more hearty items whenever you see them, I am testing this now, just need to find an NPC who sells.
5 apples cooked together will net 50 rupees, which will pay for both of the Hearty Truffles the General store sells in Hateno Village, selling for 24 rupees each. Not a bad trade? One item that that heals for 5 for two items that heal full with some change after cooked each. Sadly I have not found anyone else(not super far yet)… aside from Beedle who will sell Hearty Lizards for 100 rupees each… I mean, still worth it but not my tastes. If you know of any other shops that sells hearty materials at a reasonable price, comment it. I'll add it.