In a large saucepan, melt the butter over medium heat. Add the chopped onion and garlic, sauté until translucent.

Add the asparagus pieces and diced potatoes, stir for a few minutes.

Pour in the vegetable broth and bring to a boil. Reduce heat and simmer for about 20 minutes, until the vegetables are tender.

Using a blender, puree the soup until smooth. Return to the saucepan.

Stir in the heavy cream, salt, pepper, and lemon juice. Heat gently before serving.