Start by boiling a pot of water for the Gemelli pasta. Once it is bubbling away, toss in the pasta and cook until it is al dente. You will know it is ready when it is firm to the bite but not hard. Remember, overcooked pasta can turn mushy in your salad later.
After cooking, drain the pasta and give it a quick rinse under cold water. This helps stop the cooking process and cools it down for your salad. Just be careful not to let it sit too long, or it might stick together.
In a large bowl, combine the drained tuna, finely diced red onion, and sweet peas. You will notice the fresh aromas of the onion mixing with the tuna. This step builds up the flavor layers. Keep an eye out for any bits of tuna that have not broken up; you want it well distributed.
Gently fold in the rinsed pasta with your mixture. It should feel cozy but not overly packed. A common mistake is being too rough, which can break the pasta and create a mushy texture.
Now, add in the mayonnaise along with the salt and black pepper. Stir everything together until it is evenly coated, and the mix looks creamy. If it feels too dry, adding a bit more mayo would help avoid dryness when serving.
Let your salad chill in the fridge for at least an hour. Allowing it to rest ensures all the flavors really blend. Check it right before serving, and do not hesitate to tweak the seasoning if it tastes a bit flat after chilling.