Add diced tomatoes to a medium saucepan with a pinch of salt and black pepper.
Place pan over medium-high heat. Stir and simmer for at least 12-15 minutes until tomatoes cook down, forming a thicker paste but with some liquid. Set aside while you make the roux.
Place a cast iron skillet over medium-high heat. Melt the bacon grease or butter in a cast iron skillet.
Stir in the onion, garlic, and thyme. Saute for about 5 minutes until the onion is translucent. Add flour, cook stirring for 1-2 minutes until the drippings or butter absorb it all and the mixture is slightly golden. Stir constantly.
Pour chicken broth into the skillet, reduce heat to medium-low, and mix until completely incorporated, all lumps are gone, and gravy begins to thicken.
Add stewed tomatoes. Stir well until the gravy is thick enough to spread over biscuits. If the gravy is too thick, add water or broth two tablespoons at a time until you achieve the desired consistency.
Add heavy cream and mix well.
Taste the gravy, and adjust salt and pepper to taste if needed. Serve tomato gravy over hot buttermilk biscuits, rice, or mashed potatoes.