4 tablespoons olive oil
2 cups chopped onions
2 pounds Yukon Gold potatoes, peeled, cut into 1-inch pieces
8 cups (or more) water
1/2 teaspoon dried crushed red pepper
4 garlic cloves, minced
1 bunch mustard greens (about 12 ounces), stems trimmed, leaves coarsely chopped
1 10-ounce package fresh spinach, stems trimmed
Heat 2 tablespoons oil in heavy large pot over medium heat. Add onions and sauté for about 8 minutes until tender and golden. Add potatoes and sauté for 3 minutes. Add 8 cups water and crushed red pepper. Bring to boil. Reduce heat. Simmer for a bout 20 minutes until potatoes are tender.
Meanwhile, heat the remaining 2 tablespoons of oil in another heavy large pot over medium heat. Add garlic; sauté until fragrant, about 1 minute. Add mustard greens and all but 1 cup spinach leaves; sauté for about 3 minutes until wilted.
Add sautéed greens to potato mixture. Working in batches, purée soup in blender until smooth. (Can be prepared 1 day ahead. Cool. Cover and refrigerate.) Return soup to pot. Bring to simmer, thinning with more water, if desired. Season soup to taste with salt and pepper.
Cut remaining 1 cup spinach leaves into 1/3-inch-wide slices. Ladle soup into bowls. Add dollop of sour cream to each bowl. Garnish soup with sliced spinach leaves and serve. Serves 8.