Preheat your oven to 425°F and position two racks in the oven, one in the upper-middle position and one in the lower-middle.
Add the onions, bell pepper, and oil to a microwave-safe bowl. Cook on the highest power for 2-3 minutes, stirring every 30 seconds, until the onions are softened and the mixture is fragrant. Set aside to cool slightly.
½ yellow onion, ½ red bell pepper, 1 tablespoon neutral oil
Grab two standard cookie sheets. Using a silicone brush, spread about 1 tablespoon of melted butter onto each of those sheets. Then place three tortillas on each sheet pan, allowing about half of each one to fall over the edge.
6 burrito size flour tortillas, 2 tablespoons butter
Sprinkle the shredded cheese evenly across half of each tortilla. Then add the bell pepper/onion mixture, the beans, and about ½ cup of chicken on each one.
16 ounces monterey jack cheese, 3 cups cooked chicken, 1 ½ cups black beans
Fold the empty half of each tortilla on top, then brush evenly with the remaining two tablespoons of melted butter.
2 tablespoons butter
Place one pan on the upper-middle rack and the other on the lower-middle rack of the oven. Bake for 5 minutes, then switch the pans between racks and bake for another 4-6 minutes, until the bottom of each (the part directly touching the pan) is golden brown and crispy.
Use a spatula to carefully flip them over so the really golden brown section is on top. Slice with a sharp knife or pizza cutter, then serve hot!