This ridiculously flavorful vegetarian gravy will satisfy even the heartiest meat-eaters. Along with sherry and tomato paste, dried porcini mushrooms (available in the produce sections of large supermarkets) replicate the savory flavor of drippings.
In a medium saucepan, melt 2 tablespoons of the butter over medium heat. Add onion; cook, stirring occasionally, until softened but not browned, about 5 minutes. Add garlic; cook 1 minute more. Add tomato paste; cook, stirring until color deepens, about 2 minutes. Add sherry; bring to a boil. Reduce heat and simmer until reduced slightly, about 3 minutes. Add broth, dried mushrooms and thyme sprigs; bring to a boil. Simmer 30 minutes. Strain through a fine-mesh sieve into a bowl, pressing on solids (discard solids). Do not wash pan.
Melt remaining 6 tablespoons butter in the same saucepan over medium heat. Add flour; cook, whisking, 1 to 2 minutes (don't let flour brown). Add about 1/2 cup of the hot broth, whisking to blend. Add remaining broth a bit at a time, whisking until mixture is smooth. Season with pepper.