Spinach and radicchio team up in this stellar Italian vegetable dish with walnuts and Gorgonzola cheese sprinkled on top as accents. The slightly bitter radicchio is a lovely foil for the sweetness of the onion, the earthiness of the spinach, and the creamy nuttiness of the cheese. For a spark of color and added flavor, try adding 1/2 cup of sliced sundried tomatoes.
Cut the radicchio in half through the stem end and cut out the core. Slice the leaves crosswise into a 1/4-inch julienne; you should have about 2-1/2 cups.
Place the oil in a large skillet and heat over medium heat. Add the onion and cook, stirring frequently, until the onion is soft and tender and has begun to color lightly, 10 to 15 minutes. Add the garlic and cook, stirring constantly, until fragrant, 1 to 2 minutes.
Add the radicchio and spinach to the skillet and increase the heat to medium-high. Cook, stirring frequently, until the mixture wilts, then remove the pan from the heat immediately. Season to taste with salt and pepper. Transfer the mixture to a platter and top with the walnuts and Gorgonzola. Serve hot.