In a large soup pot, melt butter over medium heat.
Sauté leeks, celery, and carrots for 10 minutes.
Add zucchini, potatoes, 1 tsp salt, a few grinds of the pepper shaker, and vegetable stock. Mix well; note that the stock won’t cover all the vegetables – that's okay!
Bring the mixture to a boil, then reduce to a simmer (medium-low heat) and cover. Cook for 20-25 minutes or until potatoes can be pierced with a knife. Add frozen peas in the last few minutes.
Blend the soup until smooth. If using a standing blender, do this in batches and cool the soup if your blender can't handle hot liquids.
Return the soup to the pot, stir in the cream, and taste. Adjust salt and pepper if needed.