Cook sausage in an 8-qt. pot over medium heat, breaking up with a spoon, until cooked through and browned, about 10 minutes. With a slotted spoon, transfer sausage to a bowl lined with paper towels to absorb excess fat.
Add oil, onions, poblano chile, bell pepper, salt, and pepper to pot. Cook, stirring often, until vegetables are softened and fragrant, about 8 minutes. Add mushrooms and garlic and cook 2 minutes longer. Add 1 qt. chicken broth and the cooked sausage. Bring mixture to a simmer, then reduce heat to low.
Meanwhile, put guajillo chiles in a small saucepan with remaining 1 cup broth. Simmer until chiles have softened and turned the liquid reddish, 5 to 6 minutes. Transfer to a blender and whirl together. Strain chile mixture into pot with vegetables and sausage.
Bring to a boil. Add mussels, cover pot, and turn off heat. Let sit until almost all the mussels have opened. Check after about 5 minutes; if they need more time, give a quick stir, re-cover, and let sit another 1 to 2 minutes. Pour mixture into a large serving bowl.
* Longaniza sausage (Amelia likes El Mexicano brand) and guajillo chiles are found in most Latino markets.
Wine pairing: Ceja 2009 Pinot Noir. The dried chiles in these mussels like the earthiness of the Pinot. The wine's warm spices stand out, but with enough vibrant red fruit to handle the heat in the dish.