You can't
buy the kind of burgers I love. (Sigh!)
I dehydrate onions, peppers, celery, and mushrooms, and then grind them up together so it's a little rougher than coarsely-ground pepper. I keep this in a jar by the stove and add some when making hamburger patties. Besides adding its own flavor, it absorbs the fat as the burger cooks (fat has a lot! of flavor).
If I'm particularly ambitious (hey, it does happen, once in a great while), I'll use my hamburger press to make a thin patty, coat it with the mixture, add crumbled bleu cheese, another thin patty on top, and then it all gets smooshed together in the press. Absolute heaven!