01 -
Combine mayonnaise, ketchup, Worcestershire sauce, pepper, garlic powder and salt in small bowl until completely smooth.
02 -
Divide ground beef into exactly eight 3-ounce portions and gently shape into loose balls without overworking meat.
03 -
Preheat cast iron skillet or griddle on grill or stovetop until very hot (450-500°F).
04 -
Cut 3-inch square of parchment paper for pressing burgers and set aside nearby.
05 -
Drizzle preheated skillet with oil and arrange onion slices in eight separate quarter-cup mounds.
06 -
Top each onion mound with thin jalapeño slices and season with portion of salt and pepper mixture.
07 -
Cook onion-jalapeño mounds undisturbed for 30-60 seconds until beginning to brown.
08 -
Place one beef ball on each onion mound and cover with parchment paper square.
09 -
Press firmly with spatula or burger press to create 4-inch wide ultra-thin patties.
10 -
Season flattened patties with remaining salt and pepper mixture.
11 -
Cook undisturbed for 2-3 minutes until edges become crispy and browned.
12 -
Carefully flip patties so onion-jalapeño mixture is on top.
13 -
Place cheese slices on four of the patties and cook 1.5-2 minutes until well browned.
14 -
Stack plain patties on cheese-covered ones to create four double burgers.
15 -
Spread sauce on toasted buns and serve burgers immediately while hot.