Combine all ingredients in a nonreactive bowl and store in an airtight jar in the refrigerator. This sauce will keep for several months. You can replace beer with lemon juice or liquor. You can also use the Dijon style mustard for the American mustard.
My preference is to use just 2 cups of prepared mustard, keep the other ingredients the same and substitute 2 tablespoons of Real Lemon for the flat beer.
When ready to use, paint the mustard sauce on the meat or chicken and then put your favorite spice rub on. Grill or smoke.