Heat 1 tablespoon of olive oil in a large frying pan over medium-high heat. Add 1 chopped brown onion and sauté for 3–4 minutes until translucent. Stir in 2 minced garlic cloves and cook for 1 more minute.

Add 1 pound of lean ground beef, breaking it apart as it cooks. Cook for 5–7 minutes until browned; drain excess fat.

Stir in 1 tablespoon of taco seasoning, along with salt and black pepper to taste. Add 1/2 cup of tomato purée and 1/4 cup of water or beef broth. Simmer for 3–5 minutes.

On one half of a flour tortilla, layer 1/3 cup of mature cheddar cheese, a scoop of the beef filling, and 1/3 cup of Monterey Jack cheese. Fold the tortilla in half.

Place the quesadilla in the same pan and cook for 2–3 minutes per side until golden brown and crispy.

Transfer the quesadilla to a cutting board and let it rest for 2 minutes. Cut into wedges and serve.