Bring a large pot of well-salted water to a boil over high heat.

Add the penne to the boiling water and stir for the first 30 seconds so it does not stick. Cook until the pasta is tender but still a little firm in the center, 10 to 12 minutes.

While the pasta cooks, set a large skillet over medium heat and melt the butter.

Add the onion to the skillet. Cook, stirring now and then, until it softens and starts to turn light gold, 3 to 4 minutes.

Add the sliced cabbage to the skillet with 1/2 teaspoon salt. Toss it with the onion and butter so the strands start to soften.

Cook the cabbage over medium heat, stirring every minute or two, until it collapses, turns glossy, and gets a few browned edges, 6 to 8 minutes.

Stir the Dijon mustard into the cabbage mixture until it is evenly mixed through, about 30 seconds.

Lower the heat to low. Add the cream cheese pieces to the skillet.

Pour 1/2 cup of the pasta cooking water into the skillet. Stir until the cream cheese melts and forms a smooth sauce with the cabbage.

Add the grated cheddar to the skillet a handful at a time, stirring after each addition, until the sauce is smooth. Keep the heat low so the cheese stays creamy instead of turning grainy.

When the penne is ready, scoop out 1 more cup of the pasta cooking water and set it aside. Drain the penne in a colander.

Put the drained penne into the skillet with the cabbage sauce. Toss until the pasta is coated.