1 tablespoon extra-virgin olive oil, plus more for garnish
1/2 pound bulk Italian hot sausage
2 large cloves garlic
1 medium onion
1 1/2 pounds baby red or Yukon gold potatoes, thinly sliced
Salt and freshly ground black pepper
1/2 cup dry vermouth or dry white wine
1 pint cherry tomatoes, halved
4 haddock or cod fillets
A handful flat-leaf parsley, chopped
1/2 lemon
Crusty bread, to pass at table