Bring water to a boil; Then add shrimp and cook for 5 mins, just until shrimp turn pink. Do not overcook. Drain and put the shrimp into iced water. Peel, devein, and roughly chop shrimp.
Melt butter In a medium saucepan, saute onions over medium heat, add shrimp, add salt and pepper, Stir in flour and Old Bay seasoning to make a roux. Add in the milk slowly and stir constantly. Allow thickening. Add celery and cook until it is soft.
Split muffins in half and bake for 5 minutes. Then top each half with a slice of tomato and a slice of cheese. Return to the oven under a preheated broiler for 30 seconds to 1 minute or until cheese is melted. Remove from the oven and spoon the shrimp mixture over toasted English muffins. Serve immediately.
Note: buy cooked shrimp from a seafood market or in the seafood department of your grocery store. So, You can eliminate a few minutes of preparation time.
You can substitute provolone cheese with cheddar cheese or Swiss cheese.