Asparagus Sauce

A plain white sauce built on asparagus cooking water rather than all milk, thickened heavily and stretched with the tips. As written it makes a thick pouring sauce; loosen it with more of the cooking liquid if you want it soup-like.

• Proportions1/4 cup flour to 1 pint liquid works out to 2 tablespoons per cup, the usual 'medium' white-sauce proportion: thick enough to coat rather than run. Use less flour if you want something looser, or keep the full quarter cup for a sauce that holds on a plate.
• Do not skip the cooking waterThe asparagus water is the only thing giving this sauce its flavour, since there is no onion, stock or cheese. Save it deliberately when you boil the spears, and salt the boiling water lightly so the liquid is already seasoned.
• Lump preventionSlaking the flour in cold milk before it meets the hot liquid is the whole technique here. Add the slurry to the boiling liquid in a thin stream while whisking, then keep it at a bare simmer for a good five minutes so the raw flour taste cooks out.
• "Butter substitute"The recipe allows oil or margarine in place of butter, which suggests it was written for households economising on butter; the method line then simply says "Add butter," treating the two as interchangeable. Butter tastes better in a sauce this bare; use 2 tablespoons of whatever fat you prefer, stirred in off the heat at the end.
• The tips go in lastAdd the tips only to warm through, a minute at most. They have already been cooked once, and further simmering in a hot thick sauce turns them olive-drab and stringy.
• SaltA full teaspoon is a lot if the asparagus water was already salted. Add half, taste at the end, and adjust.
• YieldA pint of sauce, enough for four servings over toast, poached eggs or boiled potatoes, or two bowls if thinned into a soup.