03Add the mussels to the pot. Cover tightly and cook until most of the mussels have opened, 5 to 7 minutes (discard any remaining closed mussels). Transfer to a large bowl and cover loosely with foil.
04Heat the broiler. Strain the liquid in the pot through a fine sieve into a small saucepan, pressing down on the solids with a spoon. Add the stock and bring to a simmer over medium heat. Stir in the peas and cook until just tender, 2 to 3 minutes. Whisk in the crème fraîche, lemon zest, salt and chopped fennel fronds. Cover to keep warm.
05Drizzle the bread with oil and place it under the broiler until just golden, about 1 minute. Place a slice of bread into each of two bowls. Mound the mussels on top of the bread, then ladle the broth over everything.