These Beef Fajita Quesadillas come together quickly and are perfect for busy weeknights. The key is to slice the beef thinly against the grain for maximum tenderness and to cook the vegetables until they're just softened but still have some bite. Don't overstuff your quesadillas or they'll be difficult to flip - a thin layer of filling works best for even cooking and easy handling.

1. Step 1: Marinate the beef

   In a medium bowl, combine 1 tablespoon olive oil, lime juice, minced garlic, chili powder, cumin, 1/2 teaspoon salt, and black pepper. Add the sliced beef and toss to coat evenly. Let marinate for at least 10 minutes (or up to 2 hours in the refrigerator).

2. Step 2: Cook the beef

   Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add the marinated beef in a single layer (work in batches if needed to avoid overcrowding). Cook for 2-3 minutes per side until browned and just cooked through. Transfer to a plate and cover loosely with foil.

3. Step 3: Cook the vegetables

   In the same skillet, add the remaining 1 tablespoon olive oil. Add the sliced bell peppers and onions with the remaining 1/2 teaspoon salt. Sauté for 5-6 minutes until softened and slightly charred at the edges. Return the beef to the skillet and toss everything together. Remove from heat.

4. Step 4: Assemble the quesadilla

   Wipe the skillet clean and return to medium heat. Place one tortilla in the skillet. Sprinkle about 1/4 cup of cheese evenly over half of the tortilla. Add about 1/4 cup of the beef and vegetable mixture on top of the cheese. Sprinkle another 1/4 cup of cheese over the beef mixture. Fold the empty half of the tortilla over to create a half-moon shape.

5. Step 5: Cook the quesadilla

   Cook the quesadilla for 2-3 minutes until golden brown on the bottom. Carefully flip and cook for another 2 minutes until the second side is golden and the cheese is completely melted. Transfer to a cutting board.

6. Step 6: Make remaining quesadillas

   Repeat with the remaining tortillas and filling to make a total of 4 quesadillas. Cut each quesadilla into 3 wedges.

7. Step 7: Serve

   Serve immediately with sour cream, salsa, and chopped cilantro on the side.

For a complete meal, serve these quesadillas with a simple side salad dressed with lime juice and olive oil. If you have leftover filling, it can be refrigerated for up to 3 days and makes a great topping for nachos or filling for tacos.