Preheat oven to 180 °C. Melt butter in a pan and sauté mushrooms until soft. Season with salt and pepper. Put bagels in the oven for 5 minutes. Cut open the warm bagels and put fresh rocket on bottom halves. Put onion rings on top of rocket and spoon over the fried mushrooms. Put 2 slices of brie on top and cover with the other bagel halves.