Ingredients

• 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
• Sour cream

Instructions

1. Heat 2 tablespoons oil in heavy large pot over medium heat. Add onions and sauté until tender and golden, about 8 minutes. Add potatoes; sauté 3 minutes. Add 8 cups water and crushed red pepper. Bring to boil. Reduce heat. Simmer until potatoes are tender, about 20 minutes.
2. Meanwhile, heat remaining 2 tablespoons 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 until wilted, about 3 minutes.
3. 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.
4. 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.