2 tablespoons butter
1 leek, trimmed and finely sliced
1 clove garlic, crushed
1 teaspoon finely chopped thyme
2 tablespoons flour
2 teaspoons dry English mustard
1 tablespoon Worcestershire sauce
½ cup beer
½ cup milk or cream
125 grams grated mature cheddar cheese
1 egg yolk
sea salt and freshly ground pepper
8 large flat mushrooms, stalk removed
small knob of butter
4 long slices sourdough bread
Melt the butter in a sauté pan and cook the leeks, garlic and thyme until tender. Add the flour and cook for 1 minute. Stir the dry mustard and Worcestershire sauce together to make a smooth paste then combine with the beer and cream. Stir into the leeks and simmer gently for 5 minutes. Remove from the heat and stir in the cheese, egg yolk and Tabasco to taste. Season.
Preheat the oven to 180°C.
Mushrooms: Place the mushrooms on a baking tray and put a small piece of butter in the centre of each. Season and roast until tender. Once cooked, transfer to a paper towel-lined tray to absorb excess moisture.
Preheat the grill to high.
Toast the bread and spread each piece with some of the rarebit sauce, taking it right to the edges. Slice the mushrooms thickly and pile on top. Spoon the remaining rarebit sauce over the top of the mushrooms. Grill until bubbling and golden brown. Serve immediately with a green salad.