Set the brie on the counter so it softens while you work. Slice the red onion as thin as you can and rinse the arugula. Having everything ready makes assembly smooth and stress-free.
Toast all eight slices until they turn golden and crisp. You can use a toaster or a hot oven, whichever you prefer. Crisp bread gives the sandwich a sturdy base and a satisfying crunch.
Heat the olive oil in a skillet over medium heat. Add the turkey slices and saute for about two minutes, flipping once. You only want to warm them through, so don't let them dry out.
Spread mayonnaise on the inside of every toasted slice. Then add a layer of cranberry sauce on four of the slices. This pairing adds creamy and tangy notes in the very first bite.
Place the thin onion slices over the cranberry sauce on each base slice. Add the warm turkey on top, then pile on the fresh arugula. Stack the layers neatly so each bite has a bit of everything.
Spread a generous layer of soft brie on the remaining four toast slices. The warmth from the toast helps the cheese turn silky and creamy. Don't skip the room-temperature step, because cold brie tears the bread.
Close each sandwich with the brie-topped slice, then press down gently. Cut in half with a sharp knife so the layers show. Serve right away while the turkey is warm and the toast is still crisp.