Start by heating butter and olive oil in a skillet over medium heat. Add the sliced onions and a pinch of salt. Stir occasionally and let them cook slowly until they turn golden brown and soft. This step brings out their natural sweetness.
While the onions cook, slice the ciabatta rolls in half. Prepare your turkey slices and cut the brie cheese into thin pieces. Keeping everything ready makes assembly quick and easy later on.
Lightly toast the ciabatta rolls in a toaster or on a grill pan. This step adds a crisp texture and prevents the bread from becoming soggy once you add the fillings.
Spread blueberry jam evenly on one side of the ciabatta. Layer the turkey slices on top, followed by the brie cheese. Add the warm caramelized onions to complete the filling.
Place the assembled sandwich back on a pan or grill for a few minutes until the brie melts slightly. Once warmed through, remove, slice, and serve immediately for the best flavor.