Place onion and garlic in bottom of 6-quart slow cooker. Peel sweet potatoes, carrots and parsnip; cut into 1 inch pieces. Transfer vegetables to slow cooker and top with curry paste; sprinkle with salt.
Add broth, 2 cups water and tomatoes with juice; stir until c combined.
Cover; cook on low 8 hours or until vegetables are tender. Let cool slightly.
In batches, in blender, puree soup until smooth - or use on immersion blender in slow cooker. If using blender, transfer all soup back to slow cooker. Cook on low until heated through-about 15 minutes.
Meanwhile, arrange bread on baking sheet; broil, flipping once, until toasted about 1 minute per side. Ladle soup into bowls, dollop with yogurt. Serve with toasts. If desired, garnish with microgreens and chiles.