3. Step
Heat butter and oil in a Dutch oven over medium-high heat. Add shallots and cook, stirring often, until softened, about 2 minutes. Add white mushrooms and cook, stirring often, until they start to brown, 8 to 10 minutes. Add the porcinis, celery, sage, salt and pepper and cook, stirring often, until beginning to soften, about 3 minutes. Sprinkle flour over the vegetables and cook, stirring, until the flour is incorporated, about 1 minute. Add sherry and cook, stirring, until most of the sherry has evaporated, about 1 minute.