Melt butter in a Dutch oven over medium heat. Add onion, and sauté until tender, about 4 to 6 minutes.
Add the stock and bring to a boil. Add peas, and cook until just tender, about 4 to 6 minutes.
Remove from heat and stir in spinach and mint. Cool slightly. Stir in avocado and lemon juice.
Purée soup in a blender, work in batches if necessary, until very smooth, about 2-3 minutes per batch. If too thick, add water a little at a time until the desired consistency is achieved.
Season with salt and pepper and move to the refrigerator to chill until cold, at least 4 hours.
Garnish with peas, mint, and yogurt if desired.