To make the soup, heat the olive oil and butter in a large heavy-based saucepan over medium heat. Add the garlic, shallots and and leek and cook, stirring frequently, for 5-8 minutes without colouring.
Add the potato and cook for 6 minutes.
Pour in the fish stock and water and simmer for 12–15 minutes or until the vegetables are tender.
Add the peas, watercress and spinach and cook for 4 minutes, then stir in the cream and cook for 2 minutes.
Place the soup in a food processor or blender (or use a stick blender) and process until smooth. Taste and adjust the seasoning if necessary, then stir in the vermouth.
For the crumbed oysters, place the flour, egg and breadcrumbs in three small bowls. Dip the oysters into the flour, then the egg, and then the breadcrumbs, shaking off any excess.
Heat the butter and olive oil in a small frying pan. Add the oysters (in batches if necessary) and cook for 30 seconds each side or until golden.
Season with salt and pepper and squeeze over the lemon juice.
To serve, spoon the soup into bowls, add the oysters and a squeeze of lemon juice and garnish with extra watercress sprigs.