- 2 medium onions, coarsely chopped
- 1 pound crimini or white button mushrooms, trimmed, coarsely chopped
- 2 pounds ground beef (preferably chuck, 20% fat)
- 2 tablespoons low-sodium soy sauce
- 2 teaspoons kosher salt
- 1 teaspoon freshly ground black pepper
- Two (18×13″) rimmed baking sheets
- Arrange racks in upper and lower thirds of oven; preheat to 450°F. Pulse onion in a food processor until very finely chopped. Transfer to paper towels and squeeze dry, then transfer to a large bowl.
- Pulse mushrooms in food processor until very finely chopped. Transfer to paper towels and squeeze dry, then transfer to bowl with onions. (Alternatively, you can finely chop vegetables by hand.)
- Add beef, soy sauce, salt, and pepper to bowl with onions and mushrooms. Mix well with your hands, then divide into 3-Tbsp.-sized portions and form into patties about 2 1/2″ in diameter and 1/2″ thick. You should have about 26 patties. Divide between baking sheets.
- Bake patties 3 minutes, then flip and bake until cooked to desired doneness, 3–4 minutes more for medium/medium-well.
- Patties can be cooked 3 days ahead; wrap in plastic and chill, or freeze up to 6 months.