Step 1Preheat the broiler. Season the sardines generously on both sides. Arrange them on a plate and brush the skin side liberally with the mustard. Place the breadcrumbs in a wide bowl and press the skin sides of the sardines into the breadcrumbs to make a sturdy coating.
Step 2Arrange the sardines breaded-side up on a broiler pan, drizzle the breadcrumbs lightly with olive oil and broil until the breadcrumbs are well-browned and the fish is bubbling hot, 4 to 6 minutes. Serve immediately.