Heat grill to medium-high and oil the grates. Pound chicken evenly, coat with ½ cup BBQ sauce in a zip-top bag. Grill 5–6 minutes per side, brushing with extra sauce until chicken hits 160°F.

Preheat broiler. Sauté diced onions in a skillet over medium-high until soft and golden, 5–7 minutes. Remove, then cook bacon until crisp. Drain on paper towels.

Place grilled chicken in an oven-safe skillet. Brush with 3 tbsp BBQ sauce, then top with onions, bacon, tomatoes, and cheese.

Broil for 3–4 minutes until cheese is bubbly and golden.

Sprinkle with green onions and serve hot.