Sauté the Vegetables: 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. Sauté for about 5 minutes, stirring occasionally until the vegetables are tender and slightly caramelized. Transfer the mixture to a plate and set aside.

Cook the Ground Beef: In the same skillet, add the ground beef, cooking it over medium-high heat for 5–7 minutes. Break the meat into bite-sized pieces and season with taco seasoning, salt, and pepper. Ensure the beef is thoroughly cooked and drain any excess grease if needed.

Assemble the Quesadillas: On a clean surface, lay out 4 flour tortillas. Evenly distribute the sautéed vegetable mixture and seasoned ground beef over each tortilla. Sprinkle with cheddar and Monterey Jack cheese, add jalapeños and cilantro if desired, then place the remaining tortillas on top and press gently.

Cook the Quesadillas: Wipe the skillet clean, add remaining oil, and heat over medium heat. Place the assembled quesadillas in the skillet, cooking for 3–4 minutes per side until golden brown and crispy with melted cheese. Adjust heat as necessary.

Slice and Serve: Once cooked, let the quesadillas rest for a minute. Cut each into wedges and serve warm with dipping sauces.