Make the chipotle sauce: combine sour cream, mayonnaise, 1–2 chipotle peppers, 1/2 tsp salt, 1/4 tsp black pepper, 1/4 tsp onion powder, 1/4 tsp garlic powder, and 2 tsp lime juice in a blender or food processor and blend until smooth; set aside.
Mix the steak seasoning: in a small bowl, combine 1/2 tsp seasoned salt, 1/4 tsp black pepper, 1/2 tsp onion powder, 1/2 tsp granulated garlic, 1/2 tsp chili powder, and 1/2 tsp smoked paprika.
Prepare the steak: coat the sirloin with 1 tablespoon vegetable oil and rub the spice mixture evenly over all sides.
Cook the steak: heat a cast-iron or heavy skillet over medium-high heat and sear the steak until it reaches your desired doneness, about 3–5 minutes per side depending on thickness; remove and let rest a few minutes, then slice into small bite-sized strips. Wipe the skillet clean.
Toss the cheeses together in a medium bowl.
Assemble and cook quesadillas: heat the cleaned skillet over medium-high. Place one tortilla in the skillet, sprinkle 1/2 cup shredded Colby-Jack/mozzarella mixture over half the tortilla, add a quarter of the seasoned steak, and drizzle with chipotle sauce. Fold the tortilla into a half-moon.
Cook until the bottom is golden and the cheese begins to melt, about 1–2 minutes, then flip and cook the other side until crispy and cheese is fully melted; transfer to a plate and let rest 1–2 minutes before cutting. Repeat with remaining tortillas and fillings.