3 tablespoons tahini
1/8 cup lemon juice
1 cup plain yogurt
3/4 cup water
3 cloves garlic
1/4 cup olive oil
1 onion, chopped
2 large portobello mushrooms, sliced
1/2 red bell pepper, diced
1 (16 ounce) package penne pasta
1/2 cup chopped parsley
ground black pepper to taste