Rinse the radicchio under cold water and cut it into thick strips. In a large skillet over medium heat, sauté the onion in the olive oil. When translucent, add the radicchio and mix together for a couple of minutes. Add 1/2 cup of water, and salt and pepper. Let cook until the radicchio has wilted and the water has mostly evaporated.
Add the pieces of gorgonzola to the radicchio and mix until it has melted.
Meanwhile, bring a large pot of salted water to a boil. Cook the pasta until al dente (2 minutes less than the package recommends), and drain it, retaining some of the cooking water.
Add the pasta to the radicchio sauce and mix thoroughly for another minute. Add some of the retained cooking water if it seems dry.