Blanch the peas for 2 minutes in boiling salted water. Then plunge them into salted ice water.
Peel and finely chop the onion and garlic.
Heat some olive oil in a pot. Sauté the onion and garlic until translucent.
Add the onions, garlic, peas, and cold broth to a blender and purée until smooth. Stir in the heavy cream.
Season the soup with salt, pepper, and lemon zest.
Chill the soup for about an hour.
Crumble the feta and roughly chop the mint.
Serve the chilled pea soup with watercress, feta, and mint.