1. Melt butter in a large skillet over medium heat; add flour and salad dressing mix, stirring well. Cook 1 minute, stirring constantly. Gradually stir in milk. Add mushrooms and lemon rind; cook, stirring constantly, until thickened and bubbly.
2. Add scallops; cook, stirring occasionally, 5 to 7 minutes. Remove from heat; add cheese, stirring until cheese melts.
3. Top each English muffin half with a tomato slice; spoon 1/2 cup scallop mixture on top. Garnish with parsley, if desired.