In a 6 quart Dutch oven cook mushrooms, shallots, and carrots in hot butter about 15 minutes or until mushrooms are tender; stir occasionally. Stir in flour, thyme, garlic powder, 1/2 teaspoons each salt and black pepper and, if desired, crushed red pepper. Add broth all at once. Cook and stir until soup is slightly thickened and bubbly.
Stir in artichokes, tomatoes and bay leaf. Simmer, covered, for 15 minutes. Stir in half and half; heat through. Discard bay leaf. Top with green onions.