Prep the shrimp: Pat the shrimp dry with paper towels. Toss with chili powder, cumin, smoked paprika, red pepper flakes (if using), and a pinch of salt and pepper.
Sauté the aromatics: Heat a large skillet over medium-high.
Add olive oil and butter. When the butter foams, add the onion and bell pepper. Cook for 3–4 minutes until slightly softened and lightly charred at the edges.
Cook the shrimp: Push the veggies to one side.
Add the shrimp and garlic to the empty side. Cook for 1–2 minutes per side, just until opaque and pink. Do not overcook.
Finish with lime and cilantro: Turn off the heat.
Stir in lime juice and cilantro. Taste and adjust salt and pepper. Transfer the mixture to a bowl and wipe out the skillet.
Assemble the quesadillas: Place a tortilla in the dry skillet over medium heat.
Sprinkle a thin, even layer of cheese over half the tortilla. Spoon some shrimp mixture over the cheese. Add optional mix-ins if you like.
Top with a little more cheese. Fold the tortilla in half.
Toast until golden: Cook 2–3 minutes per side, pressing gently with a spatula, until the tortilla is crisp and the cheese is melted. If the tortilla browns too fast, lower the heat.
Repeat and serve: Make the remaining quesadillas.
Slice into wedges. Serve hot with salsa, sour cream, guacamole, and lime wedges.