Bring a large pot of salted water to boil. Add the pasta & cook, stirring occasionally, until al dente (~8-10 min). Reserve 1 cup of pasta water, drain the noodles & set aside.
Return pot to medium heat, add oil & heat through. Add butter & allow to melt then add onion & sauté until tender (~5 min). Add garlic & sauté until fragrant (~30 sec).
Stir in the Calabrian chili paste (if using) & frozen peas, stirring until the peas have heated through.
Return pasta to the pot & toss to combine, adding ½ cup of the pasta cooking water. Add tuna, lemon zest, lemon juice & parsley, tossing to combine just to break up the large pieces & heat.
Divide among pasta bowls & serve.