Pasta With Spicy Sausage, Radicchio, and Sun-Dried Tomatoes

Cook pasta in a large pot of boiling salted water, stirring occasionally, until al dente. Drain pasta, reserving 3/4 cup pasta cooking liquid; return pasta to pot. Meanwhile, heat oil in a medium skillet over medium-high. Add onion and 1/4 tsp. salt and cook, stirring occasionally, until tender, about 5 minutes (do not brown). Add sausage and cook, stirring occasionally and breaking up sausage into small bits with a wooden spoon, until sausage is mostly cooked through, about 5 minutes. Add radicchio and 2 Tbsp. water. Cook, covered, until sausage is cooked through and radicchio is wilted, 3 –5 minutes more. Add tomatoes, sausage mixture, and 1/2 cup pasta cooking liquid to pot with pasta. Season with salt and pepper, toss to coat, and add remaining 1/4 cup pasta cooking liquid, if needed. Divide among serving plates and top with pepper.