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.
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.
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.