Heat oil in a Dutch oven over medium-high heat. Add sausage and cook, stirring occasionally, until browned, about 4 minutes. Add mushrooms, red and green bell peppers, and garlic and cook, stirring occasionally, until vegetables are soft, 6 to 8 minutes. Stir in tomatoes with juices, chicken stock, marjoram, basil, salt, and red pepper flakes; bring to a boil. Reduce heat and simmer for 15 minutes. Serve hot.