Preheat the oven to 425 degrees F. Season the sole with salt and pepper. Roll the fillets up and place in a baking dish. Drizzle with olive oil and lemon juice. Bake the sole 15 minutes, or until just cooked through. While this occurs, heat the oil in a pot over medium heat. Add the onions and mushrooms and cook until tender, about 5 minutes. Mix in the flour until well combined. Slowly mix in the stock. Stir in the tarragon. Bring to a simmer and cook until sauce thickens. Stir in the whipping cream and season with salt and pepper. Divide the sole among 4 plates. Top with sauce and serve.