Preheat oven to 425°.
To prepare sauce, combine mustard and water in a small bowl; stir well with a whisk. Stir in soy sauce, plum sauce, sherry, and ginger; set aside.
To prepare lobster, make a lengthwise cut through the top of each lobster shell using kitchen shears, cutting to, but not through, lobster meat; press shell open. Place the lobster tails, cut sides up, in a shallow roasting pan coated with cooking spray. Combine the oils and pepper, and spoon over the lobster meat.
Bake at 425° for 13 minutes or until the lobster meat turns opaque. Serve lobster with sauce, and garnish with onions, if desired.