Pat the steak slices dry with paper towels. Season both sides with 1/4 teaspoon salt, black pepper, and the smoked paprika.
Add 1 tablespoon olive oil to a large skillet over medium heat until shimmering.
Add the steak slices in a single layer, working in batches if needed. Sauté for 4-5 minutes per side until browned and cooked through to your liking. Remove steak from the pan and set on a plate to rest.
In the same skillet, add the diced red bell pepper and sliced onion. Sprinkle with the remaining 1/4 teaspoon salt and cook for about 5 minutes, stirring often, until softened and lightly caramelized.
Stir in the minced garlic and cook for 30 seconds until fragrant. Remove the skillet from heat to avoid burning the garlic.
Lay out the tortillas on a clean surface. Sprinkle half of the mozzarella and cheddar evenly over two of the tortillas. Top each with a layer of cooked steak and a portion of the sautéed vegetables. Finish with the remaining cheeses and place the other tortillas on top to form two quesadilla sandwiches.
Heat a clean skillet over medium heat and melt half of the butter. Cook one quesadilla at a time for 2-3 minutes per side, pressing down gently with a spatula, until the tortilla is golden brown and the cheese is fully melted. Repeat with the second quesadilla, adding the remaining butter as needed.
Transfer cooked quesadillas to a cutting board and let them rest for one minute. Cut each into wedges and sprinkle with chopped parsley before serving.
Notes
For extra flavor, add a dash of hot sauce or sliced jalapeños in the assembly step. Store leftovers in an airtight container in the refrigerator for up to 3 days.