A glorified cheese on toast that is entirely its own thing — a rich, savoury sauce of mature cheddar melted with ale, mustard, Worcestershire sauce, and egg yolk, spread thickly on toast and grilled until bubbling and browned; one of the great British dishes, far more than the sum of its parts.
Not cheese on toast
Welsh rarebit is frequently dismissed as “just cheese on toast” by people who haven’t tasted a properly made version. The difference is the sauce: rather than melted cheese, rarebit uses a cooked sauce of cheese melted into ale with English mustard, Worcestershire sauce, and egg yolk. This produces a smooth, rich, intensely flavoured mass that is thick enough to spread and that grills to a distinctive puffed, blistered, golden-brown surface quite unlike plain melted cheese.
The debate
The name “rarebit” has puzzled people for centuries — “Welsh rabbit” is an earlier name, almost certainly a historical joke (Wales being associated with poverty; a Welshman’s “rabbit” was cheese, because real rabbit was unaffordable). By the 18th century it appears in cookbooks as a sophisticated supper dish. The “rare bit” theory — that it was a “rare bit” of flavour — is probably folk etymology.
Ale and mustard
The ale in rarebit is not a large quantity — a few tablespoons — but it is essential for flavour and for loosening the cheese to a spreadable consistency. Stout gives a more robust, slightly bitter note; pale ale or bitter gives a lighter result. English mustard adds sharpness that cuts through the richness.
Buck rarebit
The addition of a poached egg on top of the finished rarebit is called “buck rarebit” — one of the most satisfying combinations in British food, with runny yolk mixing into the cheese sauce as you eat.