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

Add 1 1/2 teaspoons of the kosher salt to the boiling water. Add the rigatoni to the pot, stir well, and cook until tender with a little firmness in the center, following the package time.

While the pasta cooks, put the pecans in a large skillet over medium heat.

Toast the pecans, stirring often, until they smell fragrant and look a shade darker, 3 to 4 minutes.

Transfer the pecans from the skillet to a plate so they do not keep cooking.

Put the frozen cauliflower and 1 cup of water into the same skillet. Cover the skillet and cook over medium-high heat until the cauliflower is thawed and just tender, 4 to 5 minutes.

Remove the lid from the skillet and keep cooking until the water has almost completely cooked away, about 2 minutes.

Add 4 tablespoons of the butter to the skillet. Add the red pepper flakes now if you want a little heat.

Cook the cauliflower in the butter over medium heat, stirring every minute, until some edges are browned and the butter turns light brown and smells nutty, 4 to 5 minutes.

Finely grate the lemon zest into the skillet. Cut the lemon in half and set it aside for later.

Scoop out 1 cup of the pasta cooking water and keep it in a mug or bowl near the stove.

Drain the rigatoni in a colander and return the pasta to the empty pot.