Fill a large pot with water and bring it to a boil over high heat.

Slice the scallions thinly. Put the white and pale green parts in one small bowl and the dark green tops in another.

When the water boils, add enough kosher salt so the water tastes lightly seasoned.

Add the spaghetti to the boiling water and stir right away so it does not stick together.

Cook the spaghetti until it is tender but still a little firm in the center, using the package time as a guide.

When the spaghetti has about 2 minutes left, add the frozen peas to the pot and stir them into the water.

Scoop 1 1/2 cups of the pasta water into a heatproof measuring cup.

Drain the spaghetti and peas together in a colander. Let them sit for a moment while you start the sauce.

Put the empty pot back on the stove over low heat.

Add the butter to the pot.

When the butter melts, add the scallion whites and pale greens to the pot.

Cook the scallions for 1 to 2 minutes, stirring often, until they soften and smell sweet but do not brown.