I like every month or two buying a pound of onions, run em over the mandolin and spend an afternoon doing a nice slow caramelize on em. Reduce em down to a cup or so of dark brown amazingness, toss it in the fridge and add into a bunch of food just like I do roasted garlic.

I’ve been going for garlic and shallots lately.
In my fridge, there’s nearly always a container with garlic butter and olive oil that I make, and another with chopped shallots with salt and pepper, soaking in olive oil.