Bring a pasta pot of salted water to a boil. Dice onion and set aside. Quarter the radicchio and cut out the core at the bottom of each quarter.  Cut crosswise into thinly sliced ribbons or chiffonade.  This can be done very quickly with a slicing blade in a food processor. Set aside.

Add rigatoni to the pasta pot and cook until al dente, according to package directions. Drain rigatoni and reserve the pasta water.

While pasta is cooking, heat olive oil in a large skillet and add onion.  Cook on medium-low heat for a few minutes, then add the radicchio and cook until wilted, a few more minutes.  Add 1/2 cup pasta water and most of the walnuts and cook an additional 5 minutes.  Add cooked and drained rigatoni along with another 1/2 cup pasta water and 1/4 teaspoon salt and a few grindings of the fresh ground pepper. Stir together to combine and cook for another 2 minutes.

Remove pan from heat, add most of the gorgonzola cheese and most of parsley, saving some for garnishing. Stir together to combine and melt the cheese. To make the dish creamier, you can stir in a small amount of additional pasta water. Serve on plates, garnishing with the reserved walnuts, gorgonzola and parsley.