Clean, trim and chop the mushrooms, then heat a skillet and saute them in the olive oil and a tablespoon of butter over medium low heat – about 5 minutes, then set aside and keep warm
Make the sandwiches by covering a slice of brioche with remoulade, pile on half the mushrooms and the grated, mixed cheeses, then top with another slice of brioche buttered on top. Repeat for the second sandwich
Heat a cast iron skillet with 2 pats of butter until it bubbles, then place the sandwich in the skillet and grill over medium low heat until it is golden brown – 1-2 minutes. Turn and place the buttered side down into the skillet and continue to grill until it is also golden brown and the cheese is melted