Toast sesame seeds until golden. Cut onion half lengthwise into thin slices. Cut bell peppers into 1-inch pieces. Mince garlic. Peel and quarter tomatoes. Seed quarters and halve lengthwise. Cut flaps and tentacles from squid sacs. Cut sacs into 1/4-inch-thick strips and crosswise into 1/4-inch-thick rings. Halve tentacles and flaps lengthwise if large. Pat squid dry with paper towels and season with salt and pepper.
Heat a wok or large heavy skillet over high heat until a bead of water dropped on cooking surface evaporates immediately. Add 2 tablespoons peanut oil, swirling wok or skillet to coat evenly, and heat until hot but not smoking. Add onion and stir-fry until softened. Add peppers and stir-fry until crisp-tender, about 5 minutes. Add garlic and stir-fry until fragrant, about 30 seconds. Add tomatoes, soy sauce, and vinegar and simmer, stirring, 1 minute. Transfer mixture to a large bowl.
Add 1 tablespoon peanut oil to wok or skillet and heat until just smoking. Stir-fry half of squid until just opaque, about 30 seconds, and transfer with juices to bowl of vegetables. Add remaining tablespoon peanut oil and stir-fry remaining squid, transferring to bowl. Return squid-vegetable mixture to wok or skillet and stir-fry until heated. Season stir-fry with salt and pepper and stir in sesame oil.
Serve stir-fry over rice, sprinkled with sesame seeds and garnished with parsley.