Clean the boletus and cut into slices. Peel and finely dice shallots. Heat oil with butter in a pan and fry the onion cubes until translucent. Rinse turkey breast fillet with cold water and pat dry with kitchen paper. Cut the fillet into thin strips, add to the onions and fry while stirring. Add brandy. Add slices of porcini mushrooms and sauté, then pour in white wine.
Fillet strips and mushrooms over medium heat, simmer in a closed pan for 5-8 minutes. Season the strips with salt and pepper. Stir in the crème fraîche and heat. Sprinkle the sliced with the tarragon leaves.