Asparagus, Potato, and Butter Bean Soup - Forks Over Knives

Ingredients

• 1 tablespoon extra-virgin olive oil (optional, <a href="#h-tips">learn more</a>)
• 3 cups sliced leeks
• 6 cups low-sodium vegetable broth
• 6 cups cubed Yukon gold potatoes (2¼ lb.)
• 2 15-oz. cans low-sodium butter beans, undrained
• 1 teaspoon mustard seeds
• ¼ teaspoon cayenne pepper
• 1 lb. asparagus spears, trimmed and cut into 1-inch pieces
• 2 tablespoons chopped fresh dill
• 1 tablespoon white balsamic vinegar
• Sea salt, to taste
• Freshly ground black pepper, to taste
• Sliced scallions or fresh chives