In a large skillet, heat 1 tablespoon of vegetable oil over medium-high heat. Add the finely chopped red onion, red bell pepper, and green bell pepper, sautéing them for about 5 minutes until tender and slightly caramelized.

In the same skillet, add the ground beef, breaking it up with a spatula. Season with taco seasoning, salt, and pepper, cooking for 5-7 minutes until browned and fully cooked.

On a clean surface, lay out 4 large flour tortillas. Evenly distribute the sautéed vegetables and cooked beef among them, then sprinkle with cheeses and cilantro.

Wipe out the skillet, add the remaining vegetable oil and heat over medium until shimmering.

Place prepared quesadillas in the skillet. Cook each side for 3-4 minutes until golden brown and cheese is melted.

Once cooked, remove from skillet, allow to cool slightly, and slice into wedges. Serve warm with sour cream, salsa, and guacamole.