Cook the onions, wine and butter in a heavy bottomed pan over a low heat, uncovered, for about 40 minutes, until the onions are soft and most of the liquid has been absorbed. Distribute the onions among the 6 warmed bowls and top with simmering stock.

Place a round of bread on top of each bowl of soup, top this with the grated cheese.

Grill for 2-3 minutes until the cheese melts into a glorious, golden brown mess. Serve immediately.