For the quesadillas:
1 lb flank or skirt steak
2 teaspoons kosher salt
1 teaspoon onion powder
1 teaspoon black pepper
1/2 teaspoon ground cumin
1/2 teaspoon garlic powder
2 tablespoons olive oil, divided
1 medium onion, sliced
2 1/2 cups shredded mozzarella cheese
2 1/2 cups shredded monterey jack cheese
10 medium (8-inch) flour tortillas
For serving (optional):
Heat a large skillet over medium-high heat. While the skillet is heating up, pat steak dry on both sides with paper towels. In a small bowl, combine salt, onion powder, black pepper, cumin, and garlic powder.
Sprinkle the seasoning evenly over both sides of the steak. Add 1 1/2 tablespoons of olive oil to the skillet and add the steak. Cook on each side for about 4-8 minutes, depending on how thick the steak is and how “done” you want it to be.
Remove the steak from the skillet and transfer it to a cutting board to let it rest. While the meat is resting, add the remaining 1/2 tablespoon of olive oil to the skillet. Add the sliced onions and cook, stirring occasionally, for about 8 minutes, until softened.
Transfer the onions to a bowl and carefully wipe off the inside of the skillet with a few paper towels until clean. Set aside.
Uncover the steak and slice it against the grain at an angle into thin slices or into bite-sized chunks.
In a medium bowl, combine the shredded mozzarella and monterey jack cheeses.
Assemble the quesadillas by adding 1/2 cup of shredded cheese to the bottom of one tortilla, followed by some steak, then onions, and one more 1/2 cup of shredded cheese. Sprinkle a pinch of salt and then top everything with one more tortilla.
Place the clean skillet back on the stove over medium heat. Using a spatula, carefully transfer the quesadilla to the skillet.
Cook the quesadillas for 1-2 minutes, or until the bottom of the quesadilla is golden-brown. Quickly flip over the quesadilla and cook for another 1-2 minutes, until golden-brown and the cheese has fully melted.
Assemble and cook the remaining quesadillas. Serve immediately as is, or cut the quesadillas into fourths. Serve with salsa, guacamole, and sour cream.