Add olive oil to large pan along with red pepper if using and saute about 1 minute over medium heat until fragrant.
Then add diced onion and continue cooking for about 3 or 4 more minutes OR until onion becomes translucent and starts to darken just a little bit.
Add frozen peas (or canned if using) to pan and heat for about 3 minutes until peas have defrosted and are tender.
If you like peas that are very tender and are using frozen fresh peas, I recommend cooking the peas longer (low and slow), up to 10 minutes over very low heat (keep covered to trap steam/moisture). Add salt and pepper to taste.
Add ⅓ to ½ cup water if the mixture dries out but also to help tenderize peas if cooking longer.
Add pasta to pan and then pour just enough water into the pan to barely cover the pasta and peas.
Give everything a good stir, cover, bring to a boil and cook pasta according to instructions (typically 9 to 11 minutes). You may uncover or place lid ajar after the mixture comes to a boil.
When pasta is finished cooking, ladle pasta with peas into bowls, and add extra broth if desired.
Optional: Drizzle with additional olive oil, fresh black pepper, grated cheese, and/or toasted bread crumbs.