When the water comes to a full boil, add 1 1/2 teaspoons of the kosher salt.

Add the spaghetti to the boiling water. Stir for a few seconds so the noodles do not stick together, and cook until tender with a little bite left in the center, 9 to 11 minutes.

While the pasta cooks, put the bacon pieces into the large skillet and set the skillet over medium heat.

Cook the bacon, stirring now and then, until the fat has melted out and the bacon is browned and crisp at the edges, 6 to 8 minutes.

Use a slotted spoon to transfer the bacon to a plate, leaving the bacon fat in the skillet.

Add the sliced onion to the skillet. Cook over medium heat, stirring often, until the onion softens and starts to turn golden, about 3 minutes.

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

Add the sliced cabbage to the skillet along with the remaining 1/2 teaspoon kosher salt. Toss it with tongs so it gets coated in the bacon fat.

Cook the cabbage over medium heat, tossing every minute or so, until it wilts down and looks glossy, about 5 minutes.

Add the black pepper and red pepper flakes to the skillet. Keep cooking until the cabbage is fully tender and lightly browned in spots, 3 to 4 minutes more.

Scoop 1 1/2 cups of the pasta cooking water out of the pot and pour it into a heatproof measuring cup.