Heat olive oil in a large saucepan over medium heat. Cook and stir onion in hot oil until softened, about 2 minutes. Stir in beans and corn, then add salsa, brown sugar, and red pepper flakes; mix well. Cook until heated through, about 3 minutes. Remove from heat.
Melt about 1 ½ teaspoons butter in a large skillet over medium heat. Place a tortilla in the skillet. Sprinkle with ¼ of the Monterey Jack cheese, then top with ¼ of the bean mixture. Place another tortilla on top; cook until the bottom is golden, then carefully flip and cook until the other side is golden. Repeat with remaining tortillas and filling.
