Melt the butter in a medium pot over medium heat. Add the onion and cook, stirring occasionally, until soft and translucent but not browned, 6β8 minutes.
Add the potato and the vegetable stock. Bring to a simmer, then cook until the potato is tender when pierced, 10β12 minutes.
Stir in the frozen peas and simmer until bright green and just tender, 3β5 minutes. Remove from the heat and add the mint leaves and the lemon juice.
Blend the soup until very smooth using an immersion blender (or carefully in batches in a blender), 1β2 minutes. Return to the pot and season with the kosher salt and the black pepper. Reheat gently over low heat until steaming; do not boil to preserve the color.
Ladle into warm bowls and drizzle with heavy cream (for serving). Serve immediately.