To make the rarebit, melt the butter in a small saucepan. Add the flour and mix them into a smooth paste. Cook gently for a few minutes until it starts to smell slightly biscuity. Add the mustard, Worcestershire sauce, and ale. Mix well and let it come to a simmer. Throw in the grated cheese and beat it together until the cheese has melted and you have a smooth thick paste. Leave to cool, transfer to a container, and refrigerate until needed.
When ready, cook the leeks gently in a frying pan with a knob of butter and pinch of salt for 10 mins or so, until softened but not coloured.
In a separate pan, fry the bacon until crispy and tear in the sage leaves right at the end so they fry and crisp up in the bacon fat.
Turn your grill to high to preheat. Toast or griddle the bread on both sides and place them on a baking tray. Top each slice equally with the bacon and leeks, Divide the rarebit mix into 4 balls and flatten them out to roughly the shape of the bread. Pop the rarebit on top, give them a turn of pepper, and slide them under the grill until bubbling and golden on top.
Serve immediately.
