Place vinegar, sugar and salt in a bowl and stir until sugar dissolves, pour over cucumber ribbons and stir through dill.
Melt butter in a saucepan over medium heat, add flour and stir for 1 minute.
Slowly add milk, whisking until smooth. Cook, stirring until thickened.
Add dijon and gruyere and stir until melted. Season to taste.
Heat oven grill to high.
Spread each slice of sourdough with béchamel and place under grill until golden and bubbly.
Top with prawns, pickled cucumber and dill and serve.