Fill a large pot with water, add 1 tablespoon of the salt, and bring it to a boil over high heat. Set a large deep skillet on a second burner.

While the water heats, finely chop the onion and celery. Grate the cheddar if needed, drain the tuna, and zest the lemon; keep the lemon halves for later.

Add the egg noodles to the boiling water. Stir once so the noodles do not stick together.

Cook the noodles according to the package directions until tender, usually 6 to 8 minutes. Keep the pot at a steady boil while you start the sauce.

While the noodles cook, put the butter in the skillet and set it over medium heat. When the butter melts, add the onion and a small pinch of the remaining salt.

Cook the onion for 3 minutes, stirring often, until it softens and starts to look clear.

Add the celery to the skillet. Cook for 3 minutes, stirring often, until the celery softens and loses its raw look.

Add the frozen peas to the skillet. Cook for 1 minute, just until they are thawed and bright green.

Scoop 1 1/2 cups of pasta water from the pot into a measuring cup. Pour 1 cup of that pasta water into the skillet and add the cream cheese pieces.

Lower the heat under the skillet to medium-low. Stir until the cream cheese melts and the liquid becomes a smooth, lightly creamy sauce.

Add the drained tuna, half of the cheddar, and all of the lemon zest to the skillet. Stir gently for 1 minute, breaking the tuna into large flakes but not mashing it.

Drain the noodles in a colander as soon as they are tender. Immediately transfer the drained noodles from the colander into the skillet.