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

Remove the sautéed vegetables from the skillet. Add the pound of ground beef, breaking it up with a spatula. Stir in taco seasoning, salt, and pepper, cooking for 5-7 minutes until fully browned.

Lay out 4 large flour tortillas. Distribute the sautéed vegetable mixture over each tortilla, add the cooked ground beef, and sprinkle cheddar and Monterey Jack cheeses on top. Optionally add jalapeño slices and fresh cilantro before covering with remaining tortillas.

In the now-empty skillet, heat the remaining 1 tablespoon of vegetable oil. Place the assembled quesadillas in the skillet, cooking for 3-4 minutes until golden brown and cheese is melted. Flip for even cooking on the other side.

Remove the quesadillas from the skillet and let them rest for a minute. Cut each into wedges and serve warm with sour cream, salsa, and guacamole.