Heat a 12-inch nonstick skillet over medium heat. Add 1 t. Olive Oil or spray liberally and brown ground beef for a few minutes. Add onion and peppers, if using, and cook 3 or 4 more minutes until all beef is browned and vegetables are softened. Drain and place in a bowl to hold warm. Add taco seasoning and stir.
Wipe skillet clean, and return to medium heat. Assemble beef, cheese, and other toppings nearby stove. Put 1 t. Olive oil or spray in pan.
Lay a tortilla down, then sprinkle an even layer of cheese on it. Sprinkle beef evenly, then add other toppings as desired. Be careful not to OVERfill, as flipping will be difficult! Sprinkle another layer of shredded cheese and top with another tortilla. Using a large spatula, press down evenly on top. Cook until cheese melts enough to stick together, then flip with the largest spatula you have. Be confident!
When tortilla is browned and everything is melted, stack on a plate in the oven at 200 degrees to hold warm. Finish the other 3 quesadillas, keeping the others warm in the oven.
To serve, take each quesadilla and place it on a board. Use a round pizza cutter to divide into quarters or sixths. Serve with ketchup for kids, or a sprinkle of cilantro and your favorite toppings for grownups.