Melt butter in a small skillet over medium heat. Cook, swirling occasionally, until foam subsides and milk solids turn deep amber.

Add panko and stir constantly until evenly golden and crisp. Pull off heat immediately.

Add lemon zest and a pinch of kosher salt off heat. Toss to combine.

Spread on a plate to cool completely.

In a large wide skillet over medium heat, warm olive oil. Add anchovy fillets and cook, pressing with a spoon, until completely dissolved, about 2 minutes.

Add fennel and sea salt to the anchovy oil. Cook gently for 10 – 15 minutes, stirring occasionally, until fully softened, translucent, and sweet.

Add olives and let warm for 1 minute.

Add tomato puree, stir well, and bring to a gentle simmer. You're looking for a bare simmer: small bubbles breaking the surface, not a rolling boil. Reduce heat to low and simmer uncovered for 8 minutes.

Gently slide in tuna with all of its liquid. Fold once or twice to keep tuna in large pieces. Simmer for 5 minutes until warmed through.

Add cooked spaghetti directly to the pan. Toss to coat, adding pasta water a splash at a time to loosen and gloss the sauce. If the sauce looks tight or greasy, add pasta water a tablespoon at a time, tossing between additions, until it looks silky.

Cook together for 2 – 3 minutes. Stir in parsley.

Twirl into warm bowls and drizzle with olive oil.

Finish with brown butter breadcrumbs and flaky salt at the table.