In a small saucepan over medium-high, melt the butter. Add the onions and shallots. Cook until onions are clear, about 4 minutes.
Spoon onion mixture into a blender jar. Add peas, broth, mint, half and half, lemon juice, salt and pepper. Blend on PUREE for 2 minutes or desired consistency.
Refrigerate until ready to serve.
Swirl in yogurt before serving. Garnish with lemon zest and mint sprigs (optional).
Recipe Notes
To further improve the smooth consistency of the soup, press it through a fine mesh strainer or food mill after blending.