1. Preheat oven to 375°.
2. Place red beets, 1/4 cup water, a dash of salt, 3 thyme sprigs, and 1 rosemary sprig in an 11 x 7-inch glass baking dish; cover with foil. Repeat procedure with gold beets, remaining 1/4 cup water, dash of salt, 3 thyme sprigs, and 1 rosemary sprig. Roast at 375° for 1 hour and 15 minutes or until a knife inserted into center of a beet meets little resistance. Remove from oven; uncover. Let stand until cool enough to handle; rub skins from beets with a kitchen towel. Discard skins.
3. Coat leek with cooking spray. Place leek, cut side down, on a foil-lined baking sheet. Roast at 375° for 25 minutes or until very tender. Cool slightly. Combine leek, orange rind, and next 7 ingredients (through 1/8 teaspoon salt) in a blender. Process until smooth.
4. Place beets in a large bowl with half of vinaigrette; toss to combine. Add salad mix and remaining vinaigrette; toss gently to coat. Place 1 2/3 cups salad on each of 6 plates. Top each serving with 1 1/2 teaspoons yogurt; sprinkle each with 1/2 teaspoon sunflower seeds.