6 leeks, (about 1 1/2 pounds total, white and pale green parts only)
3 tablespoons olive oil
2 small boiling potatoes (about 1/2 pound)
3 cups chicken broth
3 cups boiling water, plus additional to thin soup
1 cup shelled fresh peas (about 1 pound in pods) or thawed frozen peas
1/2 pound sorrel
2/3 cup yogurt
2 teaspoons fresh lemon juice, or to taste
Chopped hard-boiled egg, for garnish
Thin strips sorrel, for garnish