1. Preheat the oven to 425°. Pierce the potatoes with a fork and rub with oil. Bake for 1 hour, or until tender when pierced.
2. Meanwhile, in a large, deep skillet, melt the butter in the oil. Add the mushrooms, season with salt and pepper and cook over high heat, stirring occasionally, until tender, 20 minutes. Add the onion and garlic and cook, stirring, until the mushrooms are deeply browned, 8 minutes. Add the wine and cook until evaporated. Stir in the stock, tarragon and thyme and bring to a boil. Season with salt and pepper.
3. Slit the potatoes and fluff the insides with a fork. Season with salt and transfer to plates. Spoon the mushroom ragù onto the potatoes and serve.