1. Remove and discard root ends and dark green tops of leeks. Cut in half lengthwise, and rinse thoroughly under cold running water to remove grit and sand. Thinly slice leeks crosswise.
2. Cook pasta according to package directions; drain, reserving 1 cup cooking liquid. Keep pasta warm.
3. Process walnuts, parsley, thyme, and 3 garlic cloves in a food processor 20 seconds or until finely chopped. With processor running, pour 1/2 cup olive oil through food chute; process until well blended and a pesto sauce forms. Add 3/4 teaspoon salt and 1/2 teaspoon pepper, and process 5 seconds.
4. Sauté red bell peppers and next 3 ingredients in 3 tablespoons hot oil in a large skillet over medium-high heat 8 minutes or until crisp-tender.
5. Toss together pasta, pesto sauce, bell pepper mixture, and cheese. Stir in enough reserved cooking liquid to moisten (about 1 cup). Season with salt and pepper to taste. Serve immediately.
Tip: Place cut mozzarella cubes in the freezer for about 10 minutes before adding them to hot pasta to avoid getting a stringy, taffy-like texture when the two are mixed. You may substitute fontina or provolone cheese for the mozzarella.