Bring a large pot of salted water to the boil. Add the pasta and cook, stirring occasionally, until al dente, about 8 to 10 minutes. Reserve 1 cup of the cooking liquid, then drain the noodles and set aside.
Return the large pot to medium heat, add the oil and heat through. Add the butter and allow to melt then add the onion and sauté until tender, about 5 minutes. Add the garlic and continue to sauté until fragrant, about an additional 30 seconds.
Stir in the Calabrian chili paste (if using) and frozen peas, stirring until the peas have heated through.
Return the pasta to the pot and toss to combine, adding ½ cup (118g) of the pasta cooking water. Add the tuna, lemon zest, lemon juice and parsley to the pot, tossing to combine just to break up the large pieces and heat.