In a small pan, soften the white parts of spring onion in the butter, then stir in the flour for 1 min. Very gradually, stir in the beer to make a thick sauce( like when making a white sauce).
2
Take off the heat and stir in green parts of the spring onion, the egg, breadcrumbs and cheddar. Season and chill until you're ready to use.
3
Lightly toast crumpets, and heat grill to high. Arrange crumpets on a baking sheet, pile rarebit mixture on top and grill for a few minutes until piping hot and golden brown. 190C, 3-5min.