Place a Dutch oven or large soup pot over medium-high heat with 2 turns of the pan of EVOO, about 2 tablespoons. Add mushrooms, peppers, onion and the two smashed garlic cloves, and cook until tender, 7-8 minutes. Season with red pepper flakes, salt and freshly ground black pepper to taste.
While vegetables cook, rip the bread into small pieces and soak them in the milk in a mixing bowl for about 5 minutes, until most of the milk has been absorbed.
Add the tomatoes and chicken stock to the soup pot with the veggies and turn up the heat to bring the stoup to a bubble.