In a large, heavy-bottomed pot or Dutch oven set over medium heat, melt butter with 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 blended, smooth roux. Continue cooking until the roux begins to brown and smell nutty, 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. Be careful not to burn it.
Add sage, thyme, sherry, Worcestershire sauce, nutmeg, cayenne pepper and low sodium chicken stock, and stir to combine, then return the heat to medium. When the mixture begins to simmer, add tomato paste, and season with salt and pepper.
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.