Start the pasta: Fill a large pot with water, set it over high heat, and bring it to a boil.

Add about 2 teaspoons of the kosher salt to the boiling water.

Put the spaghetti into the pot and stir for the first 30 seconds so it does not stick together. Cook until the noodles are tender but still a little firm in the center, following the package time.

While the spaghetti cooks, pick a few fennel fronds from the top of the bulb and chop them. Set the fronds aside for the end.

Soften the fennel: Set a large skillet over medium heat and add the olive oil.

Add the sliced fennel to the skillet with 1/2 teaspoon of the kosher salt. Cook for 5 to 6 minutes, stirring now and then, until the fennel has softened and the edges look lightly golden.

Add the sliced garlic to the skillet. Cook for about 30 seconds, stirring, until it smells fragrant.

Add the capers and half of the lemon juice to the skillet. Cook for 30 seconds more so the capers warm through.

Add the drained sardines to the skillet. Use a spoon to break them into large flakes, and cook for 1 minute just to warm them through.

When the spaghetti is almost ready, scoop 3/4 cup of the pasta water into a mug or heatproof measuring cup.

Use tongs to lift the spaghetti out of the pot and put it directly into the skillet. If that is awkward, drain the spaghetti in a colander and add it right away to the skillet.

Pour 1/2 cup of the reserved pasta water into the skillet. Toss the spaghetti for 1 to 2 minutes, until the noodles look glossy and the fennel and sardines are mixed through.