1. Preheat oven to 425°.
2. Line a rimmed baking sheet with parchment paper. Place beets and carrots in a bowl. Drizzle with 1 1/2 teaspoons oil; sprinkle with 1/4 teaspoon salt. Toss. Arrange the vegetables on prepared pan. Bake at 425° for 40 minutes or until tender, stirring once. Remove from oven; cool slightly. Cut beets and carrots into 1-inch pieces.
3. Heat a Dutch oven over medium heat. Add remaining 1 teaspoon oil; swirl to coat. Add apple, onion, and garam masala to pan; cook 1 1/2 minutes. Add beet mixture, broth, and 2 cups water; bring to a boil. Reduce heat, and simmer for 30 minutes. Remove from heat, and let stand 15 minutes.
4. Place half of the beet mixture in a blender, and blend until smooth. Pour soup into a bowl. Repeat. Stir in lemon juice. Ladle about 1 1/4 cups soup into each of 4 bowls; top each serving with 3 tablespoons yogurt, 2 tablespoons walnuts, and about 1 tablespoon watercress.