Preheat oven to 425 degrees.
Arrange the cod pieces in the bottom of a covered baking dish.
Top each piece of cod with lemon slices, sliced onion and fresh tarragon.
Drizzle with olive oil and sprinkle with flaky sea salt and freshly ground pepper.
Cover and bake for 15 minutes. Uncover and bake for 5 - 7 minutes, or until cod is just beginning to flake.
I complete this meal with a warm, crusty baguette and grilled asparagus.