Melt butter in a large saucepan over medium heat.
Add onion, and sauté until tender, 4 to 6 minutes.
Add stock, and bring to a boil over high heat.
Add peas, and cook just until tender, 4 to 6 minutes.
Remove from heat, and stir in spinach and mint. Cool slightly.
Stir in avocado and lemon juice.
Pureé soup in a blender, in batches, until very smooth, about 2 minutes per batch.
Season with salt and pepper.
Chill until cold, 4 hours or up to 2 days. Thin with water as needed.
Garnish with peas, mint, and yogurt.
