Clean and trim mushrooms. Halve or quarter cremini mushrooms (about 1/2 to 1-inch pieces); leave small mushrooms whole. If using portobellos, cut into 1/2 to 1-inch cubes.In a large pot, heat olive oil and sauté onions, carrots and garlic for 3 minutes, until onions are just softened. Add mushrooms, potatoes, vegetable stock, red wine, tomato paste and rosemary sprigs; stir to mix well. Bring to a boil, reduce heat and simmer, covered, until potatoes are tender but still firm, 10 to 15 minutes.During the last five minutes of cooking, remove and discard rosemary sprigs. In a small bowl, combine cornstarch or arrowroot with about 1/2 cup of vegetable stock from the pot; stir to mix well, making sure all lumps are dissolved. Stir mixture back into the pot and cook for 5 minutes longer, until sauce thickens. Season to taste with salt and pepper.Transfer to individual serving bowls and sprinkle generously with parsley. Serve hot.