Add garlic, lemongrass, ginger and red pepper flakes and mix in. Cook until fragrant, about a minute, then stream in the broth/cornstarch mixture and add most of the other ingredients: star anise, cinnamon, shiitakes, soy sauce, tomatoes and fresh black pepper. Stir often for the first 10 minutes or so, until the cornstarch has thickened the broth a bit. Now cover pot and bring to a boil. Once boiling, lower heat to simmer and cook covered for a good half hour, until mushrooms are completely softened.