Cook spaghetti according to package directions; drain and keep warm. Meanwhile, for sauce, in a small bowl mash avocado with a fork. Stir in yogurt, lime juice, salt, Sriracha sauce, and black pepper.
In a large skillet, heat oil over medium heat. Add onion; cook 3 minutes, stirring occasionally. Add garlic; cook and stir 1 minute. Add sweet peppers; cook 4 minutes more or until vegetables are crisp-tender, stirring occasionally. Remove from heat.
Stir in cucumber and yogurt mixture. Cook over medium-low heat until heated through. Serve sauce over spaghetti and top with cilantro.
2 fat, 2 vegetable, 1 starch, 1/2 lean meat