Boil the pasta: Bring a large pot of salted water to a boil. Cook pasta until just shy of al dente.
Reserve at least 1 cup of pasta water before draining.
Sauté aromatics: While pasta cooks, heat olive oil and butter in a large skillet over medium heat. Add shallot and cook 2–3 minutes until softened. Stir in garlic and cook 30 seconds until fragrant.
Add tuna: Flake in the tuna, stirring gently so some pieces stay chunky.
Warm it through for 1–2 minutes. If using chili flakes, add them now.
Build the sauce: Pour in the cream and 1/2 cup pasta water. Stir in Dijon.
Simmer gently for 2–3 minutes until slightly thickened and glossy.
Season and brighten: Add lemon zest, lemon juice, and capers. Taste and season with salt and plenty of black pepper. The sauce should be well-seasoned but not salty.
Combine with pasta: Add drained pasta to the skillet and toss to coat.
Sprinkle in Parmesan and toss again. If the sauce tightens, loosen with more pasta water a splash at a time until silky.
Finish: If using peas, stir them in now and warm for 1 minute. Turn off heat.
Add a drizzle of olive oil for shine and richness.
Serve: Top with chopped parsley, extra Parmesan, and another crack of pepper. Serve hot with lemon wedges if you like more brightness.