Bring 6 quarts if salted water to a boil in an 8 quart pot over high heat.

Meanwhile, heat the olive oil in a large skillet over medium-high heat. Scatter the sausage over the oil and cook, stirring occasionally, until lightly browned, about 6 minutes. Stir in the leeks and cook until wilted, about 2 minutes. Scatter a handful of the cabbage over the sausage and cook, stirring until wilted. Repeat with the remaining cabbage. Season lightly with salt and pepper, and stir in about ½ cup of the stock. Bring to a boil, then reduce the heat so the sauce is simmering. Cook, adding more of the stock as necessary to keep the level of liquid more or less the same, until the vegetables are very tender, about 15 minutes. Remove from the heat and cover the pan to keep the sauce warm.

Stir the rigatoni or penne into the boiling water. Return to a boil, stirring frequently. Cook the pasta, semi-covered, stirring occasionally, until done, 8 to 10 minutes.

If the skillet is large enough to accommodate the sauce and pasta, fish the pasta out of the boiling water with a large wire skimmer and toss it directly into the sauce in the skillet. If not, drain the pasta, return it to the pot, and pour in the sauce. Bring to a boil, stirring to coat the pasta with the sauce. Check the seasoning, adding salt if necessary.

Remove the pot from the heat, sprinkle the shredded Fontina over the pasta, and toss to mix. Serve immediately in warm bowls.