Melt 3 T butter in a sauté pan over medium-low heat. Add sliced mushrooms and sauté until the mushrooms are soft and lightly browned. Remove from heat and set aside. Melt 1/2 cup butter in a large heavy stock pot, add onions and sauté very slowly, stirring frequently until soft, about 10-15 minutes, without browning. Add rice, mushrooms and stir gently for a minute or two, combining well with the butter. Dissolve saffron in 2 T of heated Chardonnay, then add to the rice along with 3/4 cup Chardonnay. Add chicken broth a little bit at a time, stirring for about 30 minutes until the rice is tender and all of the liquid is absorbed.
While the risotto is cooking, prepare the prawns. Melt 2 T of butter over medium-low heat with the garlic. Stir in 1/4 cup Chardonnay followed by prawns and sauté until prawns are no longer pink and are cooked through. Add a pinch of red pepper flakes and salt and pepper to taste. Set aside and keep warm. For final prep of the risotto, stir in the two cheeses just before serving. Add salt and pepper to taste.
To serve, place a large spoonful of the risotto on to a plate or shallow soup bowl followed by 3-4 prawns fanned out on top of the risotto. If you like, you can sprinkle a little freshly grated parmesan over the top. Serves 6.