How to Make It
Prepare the serrano salsa: Toss together onion, lime juice, salt, and serrano in a bowl. Let stand 15 minutes. Immediately before building burgers, stir in tomato and cilantro.
Prepare the burgers: Combine beef, chorizo, and salt. Form into 4 (½-inch-thick) patties.
Preheat grill to high (450°F to 500°F). Place burgers on oiled grates. Grill, uncovered, until cooked through, 6 to 8 minutes, turning occasionally.
Divide shredded lettuce among bottom halves of buns, and top with burgers and avocado slices. Using a slotted spoon or fork, top with salsa; sprinkle with Cotija cheese. Top with top halves of buns.