Add toasted walnuts and garlic to the bowl of a food processor. Pulse 5 to 7 times, until the walnuts are finely chopped. Add radicchio to the bowl, and run on low for about 10 to 15 seconds.

Scrape down sides and add orange zest, orange juice, parmesan cheese, salt, pepper and red pepper flakes. Pulse a few times until combined.

With the food processor running on low, slowly drizzle the olive oil into the top until all is combined. Stop and scrape down sides. Pulse a few times to ensure everything is evenly mixed.

Taste and adjust seasoning, as needed. You might need a few extra pinches of salt to cut down on the bitterness. An additional tablespoon of freshly squeezed orange juice can be added, too.

Use immediately or store in an air-tight container in the refrigerator, up to 4 days, until ready to use.

Serving suggestion: Toss a few heaping tablespoons with some cooked spaghetti and serve with extra parmesan cheese and red pepper flakes to garnish.