Cook the pasta (and peas): Start a pot of salted water to a boil. Boil the pasta until it is just al dente. Start tasting a few minutes before the package recommends: you want it to be tender but still a little firm on the inside; usually around 7 to 8 minutes. In the last 1 minute, add the peas. Drain the pasta and peas, then rinse with cold water.
Prep the remaining ingredients: Meanwhile, thinly slice the green onions. Mince the red onion. Drain the tuna and flake it with a fork. Place them in a large bowl with the mayonnaise, white vinegar, dill, and kosher salt.
Mix: Once the pasta and peas are cooked, add them to the bowl and mix. Taste and adjust flavors if desired, adding black pepper as necessary. Serve immediately. Store leftovers refrigerated for up to 3 days: before eating, allow to come to room temperature. You may want to add a swirl of mayo or sour cream and a pinch of salt to refresh the texture and flavor.