Cook the spaghetti or rice noodles according to package instructions. Drain and set aside.
In a large skillet or wok, heat the vegetable oil over medium heat.
Add the minced garlic and red pepper flakes to the skillet, sautéing for about 30 seconds until fragrant.
Stir in the soy sauce, chili paste or sriracha, rice vinegar, and sugar. Mix well and let it simmer for 1-2 minutes.
Add the sliced bell peppers and snap peas to the skillet, cooking for another 3-4 minutes until the vegetables are tender-crisp.
Toss the cooked noodles into the skillet, mixing everything together until the noodles are well coated with the sauce.
Stir in the chopped green onions and season with salt and pepper to taste.
Serve hot, garnished with sesame seeds if desired.