Preheat the grill to high for 5 minutes. Melt the butter in a saucepan over a medium heat, when foaming add the flour and cook for 1-2 minutes until just softened. Stir in the mustard, Worcestershire sauce and ale, then stir in the cheese until melted. Remove from the heat and allow to cool a little. Beat in the egg yolks and season to taste with black pepper.
Lightly toast the bread, scatter over a few slices of onion, then divide the rarebit mixture over the top. Arrange the rarebits on a baking sheet lined with foil. Place under the grill for 1-2 minutes until golden and bubbling. Serve immediately.