Burger Sliders with Secret Sauce

Preheat oven to 350°. In a large bowl, mix beef, eggs, 1/2 cup minced onion, garlic powder, salt and pepper lightly but thoroughly. Place meat mixture on a large parchment-lined baking sheet; shape into two 6x8-in. rectangles, each about 1/2 inch thick. Bake until a thermometer reads 160°, 15-20 minutes., Meanwhile, without separating rolls, cut bread in half horizontally. Spread 1/4 cup of dressing evenly over bottom halves of rolls., Blot meat with paper towels to remove excess fat; top meat with cheese and return to oven. Bake until cheese has just melted, 2-3 minutes. Place meat on bottom halves of rolls; spread with remaining 1/4 cup dressing. Layer with pickles, remaining 1/4 cup minced onion and shredded lettuce; replace with top halves of rolls. Brush butter on top of rolls and sprinkle with sesame seeds; cut into sandwiches. Serve immediately.