Heat 2 tablespoons of oil in a heavy large skillet over medium-low heat. Add the onion and rosemary and sauté until the onion starts to soften, about 5 minutes. Add the mushrooms and sprinkle with salt and pepper. Sauté until beginning to soften, adding more oil if needed, about 5 minutes. Stir in the tomato paste. Add half the broth and boil until almost evaporated. Add the remaining broth and boil until syrupy. Add the cream and boil until the sauce thickens. Season to taste with salt and pepper.