Cook pasta according to package directions. 5 minutes before it’s done, add frozen peas to the pot and continue to boil. While the pasta is cooking, prepare the sauce.
Melt butter in a large skillet or saucepan on medium heat. Add onions to melted butter and sauté for 5 minutes, stirring occasionally.
Add flour and whisk to combine thoroughly. Continue to cook for 1 minute, stirring constantly.
Add broth, milk and miso paste (if you're using it) and whisk until combined. Add salt, pepper and garlic powder and bring to a low boil to thicken sauce.
Once it comes to a boil, lower the temperature and simmer for 5-7 minutes, stirring regularly. If the sauce is too thick, add a little more broth to thin it.
Add the drained pasta and peas to the sauce and stir to combine. Season to taste and stir.
Store leftovers in an airtight container in the fridge for up to 3-5 days. You may need to add some additional liquid (veggie broth or milk) when reheating.