Potatoes:

1. Cut into chunks, parboil for 7–8 minutes, drain and rough up the edges, then roast at 425°F for 30–40 minutes, turning once.

Mushroom Gravy:

1. 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.
2. Taste before adding salt because the ketchup will already contribute seasoning.
3. 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.