Add onion and celery, and garlic to the pot, reducing heat to medium-low. Cook, stirring occasionally until softened, about 5 minutes
Add mustard seeds, bay leaf, thyme leaves, and salt, stirring to coat, about 1 minute
Add the stock, potatoes, asparagus stems, and lemon juice. Bring to a boil, then reduce heat and simmer, covered, 20 minutes.
Using an immersion blender, puree the soup to a smooth consistency.
Add asparagus tips to the soup and simmer a further 5 minutes
Stir in dill, remove from heat, and serve topped with croutons
Notes
If you don't have an immersion blender, you can use a blender to puree the soup in batches. Just be careful to vent the lid to allow the steam from the hot soup to escape from the blender.You can top the soup with croutons for serving, or with Spiced Lemon Roasted Chickpeas which make a perfect gluten-free crouton substitute. The lemony crunch they add complements the intense asparagus flavour in the soup.If at all possible, use fresh dill for this recipe. If you do need to use dried dill, use 1 teaspoon of dried dill in place of the fresh dill.Because the asparagus tips are tender, they are added to the soup in the last five minutes of cooking time. However, If you prefer, you can add them along with the stalks.If the asparagus stalks are too thick and not fresh, you can end up with stringy soup. Look for thinner, fresh stalks when buying asparagus.Storage: You can keep vegan asparagus soup in an airtight container in the fridge for up to 4 days.Freeze: Allow the soup to cool completely before portioning it into airtight freezer containers. Freeze for up to 3 months. Thaw frozen soup overnight in the fridge before gently reheating in the microwave or on the stovetop over low heat.