This spicy ramen broth is enriched with soy milk, which gives it a luxurious, creamy body but without any heaviness. Be sure to use an unflavored soy milk, and check the ingredients—the best store-bought soy milks only contain whole soybeans and water. The simple toppings—juicy roasted mushrooms, raw brussels sprouts leaves, cubes of firm tofu, and crunchy sprouts—offer lots of contrast but are mellow enough to let the good broth shine.
Divide the mushrooms between two baking sheets. Drizzle with the oil and sprinkle with 1/4 teaspoon salt, the black pepper, and red pepper flakes. Use your hands to toss so that the mushrooms are well coated. Roast for 20 to 30 minutes, stirring periodically, until the mushrooms are tender and crispy around the edges.
Meanwhile, combine the dashi, chilies, and garlic in a saucepan. Bring to a simmer, then cover, remove from the heat, and let steep for 30 minutes. Strain out and discard the solids and return the broth to the saucepan.
In a large glass or measuring cup, or in the tall plastic cup that usually comes with an immersion blender, combine the soy milk and the miso. Purée with an immersion blender until smooth. (Alternatively, purée in a blender.) Pour the mixture into the broth and bring to a bare simmer. Stir in the remaining 1/2 teaspoon salt and taste for balance, adding more salt if needed.
Trim the root ends off of the brussels sprouts. Gently separate the leaves, then halve or quarter the firm cores.
Meanwhile, bring a saucepan of water to boil and salt it generously. Add the noodles, in a strainer basket or the pasta insert that comes with your stockpot if you have one, and cook until tender, usually 4 to 7 minutes for dried (or according to the package instructions), or 60 to 90 seconds for fresh. Lift out the noodles, reserving the cooking water, and rinse the noodles thoroughly under cold running water in order to remove excess starch. Quickly dunk them back into the hot water to reheat, then divide among four bowls.
Divide the roasted mushrooms, tofu, and brussels sprouts over the noodles. Ladle the hot broth over each serving and top with the tender sprouts. Serve immediately, passing the condiments of your choice at the table.