In a large, heavy-bottomed pot or Dutch oven set over medium heat, melt 8 tablespoons of the butter with the olive oil. Add garlic, onion, celery and pepper, and sauté until onion is translucent, stirring often, approximately 10 minutes.
Adjust heat to high, sprinkle the flour and whisk briskly to make a roux. NOTE: mixture will appear dry and somewhat lumpy. Add additional tablespoons of butter one at a time IF NECESSARY until you are able to smooth out the roux. Continue cooking until the mixture begins to brown and take on a nutty aroma, about 5 minutes. Reduce the heat to medium low, and continue to cook, stirring often, for an additional 20 to 30 minutes, until the mixture has turned a deep brown. The deepening of a roux is a slow process, so have your heat adjusted appropriately and be careful not to burn it.
Add sage, thyme, sherry, Worcestershire sauce, nutmeg, cayenne pepper. Slowly add 3 1/2 cups low sodium chicken stock to begin (may substitute or use in conjunction with clam juice or fish stock), and stir to combine. NOTE: add additional stock in 1/2-cup increments whisking after each addition until you reach a desired consistency. I try to make mine just a bit runnier as the gravy will thicken significantly as it continues to cook. Return the heat to medium. When the mixture begins to simmer, add tomato paste. TASTE FOR SEASONING. Add salt and pepper if required keeping in mind that the crab and shrimp will lend a degree of salt to the finished dish.
Cook for approximately 15 minutes, stirring often. Add shrimp, cook 3 to 4 minutes, then gently fold in the crab meat and simmer an additional 10 minutes to heat through. Serve ladled over creamy cheddar cheese stone-ground grits.