In a large deep skillet cook the onion, the garlic, and the rosemary in the oil over moderate heat, stirring, until the onion is softened, add the mushrooms and salt to taste, and cook the mixture over moderately high heat, stirring, for 10 minutes, or until the liquid the mushrooms give off is evaporated. Stir in the tomato paste and the wine and boil the mushroom mixture until most of the liquid is evaporated. In a small bowl stir the cornstarch into the broth, add the mixture and the Worcestershire sauce to the mushroom mixture, and bring the ragout to a boil, stirring. Simmer the ragout for 2 minutes and season it with salt and pepper.