Melt butter in large heavy casserole or Dutch oven over medium heat. Add onion and cook 3 minutes. Stir in tomatoes and cook gently, breaking up tomatoes, about 5 minutes. Let stand until about 15 minutes before serving.
Add beer to tomato mixture and heat to simmering. Reduce heat to maintain slow simmer, then stir in cheese 1 handful at a time, adding next handful when first is almost completely melted. (Do not allow to boil.).
Stir in mustard, Worcestershire, pepper sauce, and ground pepper. Mix cornstarch and the cold water until completely smooth; add to rarebit and cook, stirring constantly, until slightly thickened, about 2 minutes. Spoon over Whole-Wheat Biscuit Triangles.