In large saucepan, bring 1 1/2 inches of water to a boil over medium heat. Add beets; reduce heat to low. Cover; cook 25 to 35 minutes or until tender.
Drain beets; rinse with cold water. Peel beets; cut into 1/4-inch slices.
In small bowl, combine mustard, mayonnaise, vinegar, salt and pepper; blend well. With wire whisk, slowly beat in oil until well blended.
Just before serving, place mixed greens in large bowl. Reserve 3 tablespoons dressing. Add remaining dressing to greens; toss to coat. Divide greens evenly onto 6 salad plates. Divide sliced beets evenly onto greens. Top salads evenly with cheese and walnuts. Drizzle reserved dressing over salads.