Cook the bacon in a large sauté pan over medium-low heat until crispy. Transfer to a paper-towel-lined plate and reserve 1 tablespoon of bacon grease in the pan, discarding any excess.
Increase heat to medium-high and add the white parts of the scallions, bell pepper, and jalapeño. Sauté for 5 minutes until softened, then add garlic, chili powder, cumin, salt, and pepper and cook for 2 more minutes, stirring frequently.
Transfer the sautéed vegetables to a large bowl. Add the cooked bacon, chicken, beans, sour cream, cilantro, and green parts of the scallions, and toss until evenly combined. Season with extra salt and pepper if needed.
Preheat oven to 425°F. Lightly brush the bottom of a 13x18-inch sheet pan with melted butter. Arrange tortillas overlapping around the perimeter and two in the center to cover the entire bottom, with half the tortillas overhanging the rim.
Spread the chicken filling evenly over the tortillas, then top with an even layer of cheese. Place two more tortillas in the center, then fold the overhanging tortillas back toward the center to cover the filling. Brush the tops with melted butter.
Place another large baking sheet on top of the quesadilla to weigh it down. Bake for 20 minutes, then remove the top sheet and bake for 5 more minutes until the tortillas are lightly golden. Slice and serve warm with guacamole and other desired toppings.