Preheat the oven to 400°F (200°C). Line a rimmed baking sheet with parchment or lightly oil it. A hot oven gives the cod a gentle, even bake so the outside sets quickly while the interior stays tender.

Pat the cod fillets very dry with paper towels and season both sides with 1 tsp of the sea salt and the 1/2 tsp black pepper. Drying the surface removes moisture so the olive oil and lemon will cling and the exterior will lightly caramelize in the oven rather than steam.

Make the lemon‑herb dressing: finely mince 3 cloves garlic and combine in a small bowl with 3 tbsp olive oil, zest of 2 lemons, juice of 1 lemon (about 2 tbsp), 1/4 cup chopped parsley, 1 tbsp chopped dill and 1/4 tsp of the sea salt. Whisk to combine. This bright dressing marinates the fish and infuses it with citrus and herb aromatics — zest gives intense lemon oil, juice adds acidity to balance richness.

Brush the dressing over both sides of each fillet (reserve about 1 tbsp dressing). Let the fillets sit at room temperature for 8–10 minutes while you start the quinoa. Brief resting allows the flavors to penetrate and the fish to come closer to room temperature, which yields more even cooking.

Rinse the quinoa in a fine‑mesh strainer under cold running water for 30–60 seconds, rubbing the grains with your fingers. Rinsing removes the outer saponin coating that can taste bitter and helps the cooked quinoa taste clean and nutty.

Toast and cook the quinoa: in a medium saucepan over medium heat, add 1 tbsp olive oil and the remaining minced clove of garlic. Toast for 30 seconds until fragrant (this blooms the garlic without burning). Add the drained quinoa and toast, stirring, for 1–2 minutes — toasting concentrates the nutty aroma. Add 2 cups water plus 1 cup low‑sodium broth (or 3 cups water total) and 1/2 tsp salt; bring to a simmer, cover, reduce to low and cook gently for 12–15 minutes until the liquid is absorbed. Remove from heat and let rest, covered, for 10 minutes; then fluff with a fork and stir in the remaining 2 tbsp olive oil, remaining chopped parsley and 1 tbsp chopped dill and the juice of the remaining lemon. Resting allows steam to finish cooking the grains and yields light, separate quinoa instead of gluey grains.

While the quinoa simmers, sauté the spinach: heat a large skillet over medium heat and add 1 tbsp olive oil plus the butter. When the butter melts, add a pinch of salt and the spinach (you may need to add it in batches). Toss with tongs until just wilted (about 2–3 minutes). Add a light squeeze of lemon and a grind of black pepper. Cooking spinach briefly keeps it bright green and tender; overcooking will turn it gray and soggy.

Bake the cod: place the fillets on the prepared baking sheet and spoon the reserved dressing over the top. Put in the hot oven and bake for 10–12 minutes, depending on thickness — the fish is done when it flakes easily with a fork and reads about 60°C (140°F) at the thickest part. Baking at 400°F gives a quick, even cook so the exterior sets while the interior remains moist and flaky.

Finish and plate: divide the herbed quinoa among four warm plates, mound sautéed spinach beside each portion, and set a cod fillet on top of the quinoa. Spoon any pan juices or a small drizzle of good olive oil over the fish, scatter the remaining chopped herbs, and finish with a light pinch of coarse kosher salt and a twist of black pepper. The final salt brightens flavors and the oil adds gloss and mouthfeel.

Serve immediately with lemon wedges on the side. A final squeeze of lemon at the table brightens the whole plate and balances the olive oil richness.