Combine the sauce and stock in a saucepan and simmer until reduced by half.
Strain through a chinois (fine china cap) or a regular china cap lined with cheesecloth. Cover to prevent a skin from forming. Keep hot in a bain-marie, or cool in a cold-water bath for later use.
Per 1 fl oz (29.57 mL): Calories 25; Protein, 1 g; Fat 1.5 g (53% cal.); Cholesterol, 5 mg; Carbohydrates, 2 g; Fiber, 0 g; Sodium, 20 mg.