Treviso, Walnut, and Gruyère Salad

Treviso is a red, spear-shaped variety of radicchio.
Combining it with prewashed baby greens
makes a bowlful of holiday colors.
Keep the cheese cold until grating
so that it forms firm strips.

Ingredients:
1/4 cup white wine vinegar
1 teaspoon Dijon mustard
1/3 cup olive oil
Coarse kosher salt
1 pound Treviso (about two 8-ounce heads)
4 cups mixed baby greens or other baby lettuces
1/4 cup chopped fresh chives
1 cup (packed) coarsely grated Gruyère cheese (about 4 ounces)
1 cup walnut halves, toasted

Directions:
Whisk vinegar and mustard in small bowl to blend.
Gradually whisk in oil. Season dressing to taste
with coarse salt and freshly ground black pepper.
Cut Treviso crosswise into 1/2-inch-wide strips;
discard bottom 2 to 3 inches. Place strips in large bowl;
add mixed baby greens and chives.
DO AHEAD: Dressing and salad can be made 6 hours ahead.
Cover separately and chill.
Bring dressing to room temperature and rewhisk before using.
Pour dressing over salad and toss to coat.
Add cheese and walnuts;
toss salad, then transfer to serving bowl.