Heat oven to 400F. Meanwhile toss chopped cauliflower with olive oil, salt and pepper. Roast in heated oven for 12 minutes.
Remove pan from oven and add chopped walnuts. Return to oven for 5 minutes.
Remove pan from oven again and let cool. Meanwhile add mixed greens to salad bowl. Top with roasted cauliflower and nuts. Toss together with vinaigrette and serve.