Melt butter, whisk in rice flour, and cook for 1–2 minutes. Add stock gradually, whisking until smooth. Simmer until thickened, then stir in the mushroom ketchup and mustard.
3 Taste before adding salt because the ketchup will already contribute seasoning.
4 The gravy will provide a good second use for Wednesday's condiment, though I’d start with just 2 tablespoons and taste before adding more.