Stir together mayonnaise, ketchup, Worcestershire sauce, 1/2 teaspoon black pepper, garlic powder, and 1/4 teaspoon kosher salt in a small bowl. Cover and chill until ready to use.

Divide the beef into 8 (3-ounce) portions, shape each into a loose ball, and chill on a plate.

Cut a 3-inch square of parchment paper and set aside. Preheat your grill to high (450°F to 500°F) and place a large cast-iron skillet or griddle on to preheat.

Drizzle the skillet with oil. In batches if needed, arrange onion slices into 8 mounds on the skillet, top with jalapeño slices, and sprinkle with 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook, undisturbed, until the onions brown, about 30 seconds to 1 minute.

Place a beef ball on each onion mound. Top with the parchment square and press firmly to flatten into a 4-inch-wide patty. Sprinkle with the remaining salt and pepper and cook without moving for 2–3 minutes.

Flip the patties so the onion-jalapeño side is up. Top with a slice of cheese on 4 patties. Continue cooking until browned and melted, about 1 minute and 30 seconds to 2 minutes.

Top each cheesy patty with a plain patty to create double stacks. Place on the bottom buns and spread about 1 tablespoon of burger sauce on each top bun. Place on top and serve immediately!