1. In a large pot of boiling salted water, cook the pasta until al dente. Drain, reserving 1/4 cup of the cooking water. Transfer the pasta to a large bowl. Drizzle lightly with olive oil and toss to coat. Let cool to room temperature.
2. Meanwhile, in a medium bowl, stir the lemon juice with the lemon zest, tomato paste, garlic, shallot and thyme. Stir in the 6 tablespoons of olive oil and the reserved pasta cooking water. Season the dressing with salt and pepper.
3. Add 1/4 cup of the dressing to the pasta and toss. Add the sausages, peppers, radicchio, celery, mozzarella and the remaining dressing and toss well. Season the pasta salad with salt and pepper, sprinkle with the basil and serve.