2. For the sauce, heat the oil in a non-stick sauté pan. Add the onion and sauté over medium heat for about five minutes, or until the onions are softened, but not browned. Add grapefruit juice, broth, ground mustard and thyme. Bring to a boil and then reduce heat to low. Simmer until the sauce has reduced by half. Do not overcook or sauce may turn bitter. Remove from heat and carefully transfer sauce to a blender or food processor. Add prepared mustard and blend or process until smooth and return sauce to pan. Season to taste with salt and pepper. Cover and keep warm.