Heat olive oil in a heavy-bottomed pot over medium-high heat. Sauté onion, carrot, and celery until soft, 3-5 minutes. Season with salt. Add minced garlic and red pepper, sauté about 1 minute, until the garlic is fragrant. Add potatoes, season. Add stock, bring to a boil, then reduce heat and simmer until the potatoes are tender, 15-20 minutes. Add the spinach and dill, season, and partially cover. Simmer until the spinach has wilted, about 5 minutes. Add lemon juice and allow to cool slightly. Puree soup, using an immersion blender, blender, or food processor, until smooth. Season with additional salt and pepper, if needed.
While the soup is simmering, make croutons. Preheat oven to 400°. Toss the cubed pumpernickel with olive oil, salt, and pepper and bake until crunchy, about 10 minutes.
Ladle soup into warm bowls. Sprinkle with croutons, add a dollop of yogurt, and garnish with fresh dill.
Recipe by strawberryplum at http://www.strawberryplum.com/spinach-dill-soup-with-pumpernickel-croutons/