To make salsa: heat oil in skillet on medium high heat. Add onion and saute until softened, about 2 minutes. Add jalapeno and plums and stir together with onions. Stir in lime juice, bring to boil and remove from heat. Stir in coriander, season with salt and pepper and cool.
Cut brie into strips. Place 1/4-cup brie and 2 tb salsa on one half of tortilla. Season to taste. Fold over pressing edges together.
Heat remaining butter in skillet on high heat. Fry tortillas about 2 minutes per side until browned. Place in oven to keep warm. Alternatively, place quesadillas on oiled cookie sheet and oil tops. Place in 400F(200C) oven and bake for 10 minutes or until browned. To serve cut each tortilla into three.
These can be make ahead and reheated when needed in 350F(180C) oven until cheese melts.