Place beets in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and simmer for 30 minutes or until tender. Cool; peel and cut into 1/4-in. strips.
In a small bowl, whisk vinegar and mustard until blended; gradually whisk in oil. Add the sugar, salt and pepper. In a large bowl, combine the beets, onions, mint and 2 tablespoons vinaigrette. Cover and refrigerate for 30 minutes.
In a large salad bowl, combine the spinach, orange sections, beet mixture and remaining vinaigrette; toss. Top with raspberries. Serve immediately. Yield: 4 servings.