Preheat charcoal or gas grill for direct cooking over high heat (400°). Stir together mayonnaise, garam masala, and turmeric in small bowl; set aside.

Place chile peppers and onion slices in large bowl. Add vegetable oil, 1 teaspoon alt, and ¼ teaspoon black pepper; toss to coat.

Grill chile peppers and onion slices 10 to 15 minutes or until slightly charred, turning occasionally. Transfer onions to cutting board and chiles to large glass bowl. Cover chiles with plastic wrap. Let stand for 20 minutes or until cooled.

Remove seeds from grilled chiles. Cut chiles in half; slice half of the chiles and place in small bowl. Add half of the onion slices, lime juice, and olive oil; toss to coat and set aside. Chop remaining chiles and onion slices.

Combine ground beef blend and chopped chile-onion mixture in large bowl; do not overmix. Divide mixture evenly into 8 portions. Form each portion into a meatball; cover and refrigerate 15 minutes.

Place large cast iron griddle on grill rack to preheat. Remove meatballs from refrigerator; sprinkle with remaining 1 teaspoon salt and 1 teaspoon black pepper. Place 4 meatballs on hot griddle 4-inches apart. Sear 30 seconds; smash burgers to ¼-inch thickness using a large spatula. Cook 6 to 8 minutes or until edges begin to brown. Turn burgers; cook 4 minutes. Top with cheese and grill1 to 2 minutes more or until cheese is melted and burgers reach 165°. Transfer burgers to a tray; loosely cover with foil to keep warm. Repeat grilling with remaining 4 burgers; do not top with cheese.

Grill buns, cut sides down, 30 to 60 seconds or until lightly toasted. Remove from grill.

To serve, top bun bottoms with cheese-topped burgers, plain burgers, and sliced chile-onion mixture. Spread mayonnaise mixture on bun tops; place on top of burgers.