Vegetarians and vegans, rejoice. Everyone else, hear us out. This make-ahead vegetarian gravy is completely foolproof. First, it doesnt require turkey pan drippings, so you dont need to cook your turkey first (brilliant). Second, theres no tricky rouxjust mix in some flour while youre sauting your vegetables. Third, its as rich and flavorful as real gravy, thanks to umami-packed portobello mushrooms.