Peel and chop the onion. Heat the oil in a large pot on medium-high. Add the onions to the pot and stir to coat them with oil. Cook the onions for five minutes or until they're soft, stirring occasionally to prevent sticking. While the onions are cooking, you can grate the cheeses if needed. (I use my blender.)
Add the frozen peas, salt and pepper to the pot and stir to combine. Let the peas cook for five minutes, stirring occasionally. They should be almost fully cooked.
Add the water to the pot and increase the heat to high. Cover the pot with a lid and let the water come to a boil. When the water is boiling, add the pasta and stir it to prevent sticking. Lower the heat slightly and cook the pasta, uncovered, until it's al dente, stirring frequently to prevent sticking. (Make sure to scrape along the bottom of the pot.) When the pasta is cooked, do NOT drain the water.
Turn off the heat and stir in the Parmesan and Romano. Taste to see if you need any extra salt. (You may not, since the Romano is salty.)
Serve with extra Parmesan (or Romano) and red pepper flakes. Store leftovers in the refrigerator for up to four days. Add a little olive oil before reheating leftovers, so they're not dry.
Video
Notes
I always use baby peas, since they're sweeter than regular peas.
If you want to make this with meat, you can add 4 ounces of cubed pancetta or prosciutto or 3-4 strips of bacon, chopped, when the onions are soft.
Use store-bought, dried pasta for this recipe. We haven't tested it with homemade pasta, which cooks much faster and may not provide enough time for the water to absorb.