In a large pot of lightly salted boiling water, cook pasta 10 to 12 minutes, or until al dente.
While pasta is cooking, stir together tahini and lemon juice. Place in a food processor with yogurt, water, and garlic cloves; process until smooth.
Heat the oil in a saute pan over medium heat. Add the onion, and cook until soft. Add the mushrooms, and cook until soft. During the final few minutes of cooking add the bell pepper; the pepper should be still crispish.
Drain the pasta. Toss with the yogurt-tahini sauce, chopped parsley, and freshly ground black pepper. Serve the vegetable saute over the noodles.
