For chimichurri, whisk ingredients in a bowl and season to taste.
Preheat a char-grill or barbecue to high heat. Brush sardines with olive oil and season with sea salt. Grill until just cooked (4-5 minutes each side), then place in a single layer in a large non-reactive baking dish. Spoon chimichurri over sardines, set aside to marinate (2 hours).
Just before serving, brush bread with olive oil and grill until toasted (1-2 minutes each side). Serve sardines, drizzled with chimichurri, on ciabatta crostini.