In a 12 inch skillet over medium heat, melt 1 TBSP butter. Add shrimp shells and saute until shells are browned- about 5-7 minutes. Stir in 1 TBSP tomato paste and cook for 30 seconds. Add 2 1/4 cups of water and bring to a boil, then cover and simmer for 5 minutes.
Strain the shrimp through a fine sieve, pressing to release all the juices through. You should have about 1 1/2 cups of seafood stock. Add more liquid if needed.
Cook 3 slices chopped bacon in the same skillet- it is seasoned with good stuff. Cook bacon until crispy- about 8 minutes. Wipe clean any extra fat. Add 1 1/2 lbs of shrimp, 2 cloves chopped garlic, 1/2 cup green bell pepper, 1/4 cup celery, salt and pepper and cook shrimp until the edges are just pink- but not thoroughly cooked through- about 2 minutes. Transfer shrimp to a platter (with all juices) and keep warm while you finish the sauce.
In the same pan, melt 2 TBSP butter over medium low heat. Whisk in 2 TBSP flour to create a roux and cook for 1 minute- do not burn! Slowly add shrimp stock and whisk until all ingredients are well blended. Bring to a boil, then reduce to low and let simmer until slightly thickened about 5-7 minutes.
Stir in shrimp and all the juices and cook about 3 minutes. DO not overcook the shrimp. Add in 1 TBSP lemon juice, OLD BAY, Tabasco and season with salt and pepper to taste. Serve over grits, top with chopped green onions/scallions.