Heat olive oil in a large frying pan over medium-high heat. Add brown onion, cooking until translucent, about 3-4 minutes. Stir in minced garlic and sauté for an additional minute.

Add lean ground beef to the pan, breaking it up as it cooks. Stir frequently for 5-7 minutes until browned. Drain excess fat.

Mix in taco seasoning, salt, and black pepper to taste. Pour in tomato purée and water or beef broth. Simmer for 3-5 minutes to thicken.

Lay a flour tortilla on a flat surface. Sprinkle cheddar cheese, add savory beef filling, top with more cheese, and fold the tortilla.

In the same pan, add more oil if necessary. Cook folded quesadilla for 2-3 minutes on each side until golden and crispy.

Transfer to a cutting board, cool for a minute, then slice into wedges. Serve with your favorite toppings.