Toast coriander seeds in a dry skillet or place them in a flat dish and microwave on High for 4 to 5 minutes. Crush the mustard and coriander seeds slightly in a mortar or blender, then mix them and the powdered mustard into the water and let stand for at least three hours. Mix the remaining ingredients and pulverize in a blender until smooth. Stir the puree into the mustard. Bring the mixture to a boil, then lower the heat and simmer 5 minutes or until as thick as you like, stirring occasionally. The mustard will thicken slightly on cooling. Refrigerate, covered.