In a pan, saute mushrooms in 1/4 cup of broth over high heat for 3 mins until softened. Add the garlic, cook for 1 more minute. Transfer to a medium bowl, mix with chopped cilantro.
Cook the onion and celery in the pan with the rest of the broth for 5 more minutes. Stir in the tomatoes, chili flakes and a little salt, then bring to the boil. Reduce the heat and simmer, uncovered, for 10 mins until thickened.
Meanwhile, boil the spaghetti, then drain. Toss with the sauce, top with the garlicky mushrooms, serve.