Heat a skillet over medium-high heat and add a splash of oil.

Add the diced onion and cook until soft, about 2 minutes.

Add the ground beef, breaking it up with a spatula, and cook until browned, about 6 to 8 minutes. Season with salt and pepper. Drain any excess fat and set the meat aside.

In a second skillet, heat a little oil or butter over medium heat.

Place one tortilla in the skillet and sprinkle cheese over half of it.

Add a layer of cooked beef on top of the cheese, then add sliced pickles and drizzle with mustard.

Fold the tortilla to enclose the filling and press down gently with a spatula. Cook until golden brown, about 2 to 3 minutes.

Flip the quesadilla and cook the other side until golden and cheese is melted, another 2 to 3 minutes.

Transfer the quesadilla to a cutting board, let it rest for a minute, and slice into wedges.