Bring a large pot of salted water to a rolling boil (use about 1 tbsp salt per 4–5 liters). The pasta water should taste like the sea — salting now seasons the pasta from the inside as it cooks.

While the water heats, pat the cod fillets dry with paper towels and season both sides generously with salt and a few grinds of black pepper. Drying the fish promotes a golden crust when searing; salt seasons the flesh and helps draw out excess surface moisture.

Zest the lemon and squeeze 2 tablespoons of juice into a small bowl; finely chop the shallot and mince the garlic. Having these prepped keeps the finish quick and prevents burning the garlic while cooking.

When the pot boils, add the angel hair and stir immediately to prevent clumping. Cook until al dente (check package; angel hair typically 3–4 minutes). Reserve 1 cup (240 ml) of the starchy pasta cooking water before draining — the starch will help create the sauce and bind the oil/butter to the pasta.

Heat a large nonstick or stainless-steel skillet over medium-high heat. Add 2 tablespoons of olive oil and let it shimmer (this indicates it’s hot enough). Place the cod fillets in the pan and sear undisturbed for 2–3 minutes per side for 1" thick fillets, until a golden-brown crust forms. High heat and minimal movement produce a caramelized exterior while retaining moisture inside.

If your fillets are thicker than 1", reduce heat to medium and continue cooking another 2–3 minutes, or transfer the pan to a preheated 375°F (190°C) oven for 4–6 minutes until the internal temperature reaches 60°C (140°F). Fish continues to cook after removing from heat, so pull it just shy of complete doneness.

Remove the fish to a plate and tent loosely with foil to rest. Resting lets the juices redistribute and avoids overcooking while you finish the sauce.

Using the same skillet, lower heat to medium, add 1 tablespoon olive oil and the butter. Once the butter melts, add the shallot and sauté 1–2 minutes until translucent — this builds a sweet aromatic base. Add the garlic and red pepper flakes (if using) and cook 20–30 seconds until fragrant; garlic cooks quickly and will turn bitter if overcooked.

Add the reserved pasta water — about 1/2 cup to start — and the lemon juice to the pan and bring to a gentle simmer. The water loosens browned bits from the fish and pan (deglazing) and the starch creates a silky emulsion with the oil and butter that will cling to the pasta.

Add the cooked angel hair and frozen peas directly to the pan. Toss or gently toss with tongs for 1–2 minutes, adding more reserved pasta water as needed to make a glossy sauce that coats the strands. Finish with the lemon zest, chopped parsley, and most of the Parmesan, tossing so the cheese melts into the sauce and adds savory depth.

Taste and adjust seasoning with salt, pepper, and more lemon juice if you want brighter acidity. Heat should be low at this stage to avoid overcooking the pasta or wilting the herbs.

Divide the pasta among warmed plates, place a seared cod fillet atop each nest of pasta, and finish with a drizzle of olive oil, a sprinkle of remaining Parmesan, and a few extra parsley leaves. Serve immediately so the fish remains crisp and the pasta silky.