Chimichurri, a traditional Argentinian herb sauce, was devised to serve with steak, but it is terrific with any grilled meat or fish.
Place parsley, garlic, oregano, shallot, vinegar, red pepper flakes, salt and pepper in the bowl of a food processor fitted with a blade attachment. Process until finely chopped, stopping and scraping down the sides of the bowl with a rubber spatula as needed, about 1 minute total.
With the motor running, add oil in a steady stream. Scrape down the sides of the bowl and pulse a few times to combine. Transfer sauce to an airtight container and refrigerate at least 2 hours or up to 1 day to allow the flavors to meld. Before serving, stir and season as needed. The chimichurri will keep in the refrigerator for up to one week.
On a clean cutting board, roughly chop both the kale and the radicchio and place in a medium bowl. The pieces should be about the size of a dime. When ready to serve, pour an appropriate amount of the chimichurri over the chopped kale and radicchio and toss vigorously to coat and tenderize the kale. Adjust seasoning before serving over a grilled steak.