1 tablespoon olive oil
3/4 cup chopped leek (white and pale green parts only; about 1 medium)
1 teaspoon (scant) coriander seeds
1 1/4pounds yellow crookneck squash, coarsely chopped
1 (14-ounce) can vegetable broth
1/4 cup crème fraîche* or sour cream
1/2 teaspoon finely grated lemon peel
*Sold at some supermarkets and at specialty foods stores.