In a small bowl, combine chili powder, sea salt, black pepper, smoked paprika, onion powder, garlic powder, cumin, and cayenne (if using); mix well.
Toss the sliced red and green bell peppers, sliced red onion, and sliced jalapeños with the spice mix until evenly coated; set the vegetables aside while you heat the cooking surface.
Heat a large griddle or large cast iron skillet over medium-high heat and lightly oil the surface.
Working in batches if needed, cook the marinated flank steak 3–5 minutes per side on the hot griddle until it reaches your desired doneness (130–135°F for medium-rare, 135–145°F for medium); remove and rest briefly, then thinly slice against the grain.
On the same griddle, cook the seasoned fajita vegetables about 5–7 minutes, stirring occasionally, until onions are soft but peppers still have a slight bite; remove to a plate.
If needed, wipe the griddle surface with a paper towel to remove excess oil or bits before toasting quesadillas.
To assemble each quesadilla, fold one tortilla in half after layering: spread a layer of shredded Monterey Jack on one half, top with sliced steak and a portion of the fajita vegetables, then sprinkle shredded queso blanco and fold the tortilla over the fillings.
Place assembled quesadillas on the preheated griddle and cook 2–3 minutes per side, pressing gently, until the tortillas are toasted and the cheese is fully melted.
Make the dipping sauce by stirring sour cream or crema with chipotle vinaigrette to taste until smooth; slice quesadillas and serve with the sauce for dipping.