Heat olive oil in a large skillet over medium-high heat. Add ground beef and cook, breaking it apart with a spatula, until browned, about 5-7 minutes. Drain any excess fat.

Add diced onion to the skillet with the beef and cook until softened, about 3-4 minutes. Stir in minced garlic and cook for another minute until fragrant.

Sprinkle taco seasoning over the beef mixture, then add water. Stir well to combine and simmer for 2-3 minutes, allowing the liquid to reduce and the flavors to meld. Remove from heat and set aside.

In a medium bowl, combine the shredded Monterey Jack and cheddar cheeses.

Place one flour tortilla flat on a clean surface. Sprinkle about 1/4 cup of the mixed cheese over half of the tortilla.

Spoon about 1/4 cup of the cooked beef filling evenly over the cheese on the same half of the tortilla.

Sprinkle another 1/4 cup of cheese over the beef filling. Fold the other half of the tortilla over the filling to create a half-moon shape. Repeat with the remaining tortillas, beef, and cheese.

Wipe out the skillet used for the beef, or use a clean large skillet. Melt 1/2 tablespoon of butter over medium heat.

Carefully place one or two assembled quesadillas (depending on skillet size) into the skillet. Cook for 3-4 minutes per side, or until golden brown and crispy, and the cheese is fully melted. Press down gently with a spatula to ensure even browning.

Remove cooked quesadillas from the skillet and transfer to a cutting board. Repeat with the remaining butter and quesadillas.

Once all quesadillas are cooked, slice each one into two triangles.

Serve immediately with your favorite toppings like sour cream, salsa, guacamole, fresh cilantro, and lime wedges.