or anything left over in a can that you only need a small amount of for a recipe?
Freeze it. I use the Beaba baby food containers I froze Reed’s food in when I made it, since they have a lid. You can also use an ice cube tray-the ones with the covers are ideal, but you can just wrap it since you only need it in your freezer until it’s solid and then pop them out and put in a baggie with a label or if you have a covered container like mine, just keep it in there to be “green” and save a plastic bag. Measure out in increments that you most commonly use. For the tomato paste, I did 2 TBSP since I can easily cut the paste in half while it's still frozen in the event I only needed 1 TBSP for a recipe. Next time you need just a little tomato paste, you can just grab one out of the freezer and add to your recipe. I do this for leftover stock or broth if I don’t think I will be able to use it in the 7-10 days from opening. I do that one based on ounces.