2) Preheat a long, two-burner cast iron griddle or a large frying pan over a medium heat.
3) Lay two tortillas on a work surface and spread each evenly with the refried black beans. Place the tortillas, bean side up, on the griddle (cook one at a time if using a frying pan).
4) Sprinkle the onion-red pepper mixture evenly over the top of each tortilla, then sprinkle evenly with the cheese. Cover with another tortilla coated with refried black beans and cook until the cheese melts, about 4 minutes.
5) Flip the quesadillas to toast the other side. Slice each quesadilla into eight wedges, sprinkle with coriander and serve with the lime-coriander sour cream.
Lime-coriander sour cream:
In a small bowl, mix all the ingredients together until combined. Season, to taste, with salt if needed.