Cut the top 1 1/2 inches off the asparagus to reserve the tips, then chop the remaining stalks into 1 inch pieces. Melt 1 tbsp of butter in a large pot over medium heat and saute the tips for 3 minutes until bright green and crisp tender. Set aside.
Melt the remaining butter in the same pot. Add the onion and cook for 5 minutes until soft, then stir in the garlic for 1 minute.
Add the asparagus stalks, potato and broth. Bring to a boil, then simmer for 15 minutes until everything is very tender.
Blend the soup with an immersion blender until completely smooth and bright green.
Stir in the oat milk, lemon zest, lemon juice, salt and pepper and warm through for 2 to 3 minutes.
Serve topped with the sauteed asparagus tips and chives.