Stir together the 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 ground beef into 8 portions and shape each portion into a loose ball. Place the balls on a plate and chill while you prepare the grill.

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

Drizzle the skillet evenly with canola oil. Arrange the sliced onions in 8 small mounds (about 1/4 cup each) on the skillet, leaving at least 2 inches between them. Top each onion mound with jalapeño slices, about 1 tablespoon each. Sprinkle with 1/2 teaspoon kosher salt and 1/4 teaspoon black pepper. Cook for 30 seconds to 1 minute until the onions start to brown.

Place one meatball on each onion mound. Cover each with the parchment paper square and press down firmly to flatten the beef into a 4-inch-wide patty. Remove the parchment paper and repeat with the remaining meatballs and onion mounds.

Sprinkle the smashed patties with the remaining 3/4 teaspoon kosher salt and 1/2 teaspoon black pepper. Cook until the edges are crispy, about 2 to 3 minutes.

Flip each patty so the onion and jalapeño mixture is on top. Top 4 patties with a cheese slice and continue cooking until the cheese melts, about 1 minute and 30 seconds to 2 minutes.

Stack each cheese-covered patty onto a plain patty to create 4 double stacks. Transfer each stack to a bottom bun.

Spread about 1 tablespoon of the burger sauce on each of the top buns and place them on top of the patties. Serve immediately.