- Place butter and oil in a medium to large pot set over medium heat. When butter is melted, add onions and cook 10 to 12 minutes, or until a rich golden colour. Add garlic, rosemary and mustard and cook and stir 1 minute more.
- Add the stock and beer and bring the soup to a gentle simmer (small bubbles should just break on the surface). Gently simmer the soup, uncovered, for 25 minutes. Season the soup with salt and pepper and reserve on low heat.
- Set an oven rack 6” beneath the broiler. Set the baguette on a baking sheet. Broil the bread on both sides until lightly toasted.
- To serve, set 6 ovenproof soup bowls on a baking sheet. Ladle the soup into the bowls. Set 2 slices of toasted baguette on top of each soup. Divide and top bread on each soup with grated or sliced cheese. Broil soup a minute or two, until the cheese is melted and light golden. Sprinkle with parsley, if using, and serve.
Instead of beef stock, you could use vegetable stock or chicken stock in this recipe. When unavailable, replace the Chimay cheese with another firm and tangy type, such as Gruyere or Swiss.