Sweet scallops in a cloak of bubbling cheese. Post-grilling, the white sauce is so blotched with dark-coloured patches that it reminds me, weirdly, of cows.
Pour 1 tsp oil on to a small plate. Heat a small frying pan over a high heat. Take a scallop and dip both sides in the oil. Season with a little salt. Repeat with the remaining scallops. When the pan is very hot, add the scallops and cook for about 30 seconds on each side until crusted deep golden brown but only very lightly cooked. Transfer to a plate.
Melt the butter in a small pan and whisk in the flour with a small, metal whisk. Cook the flour for 30 seconds or so but do not burn it or you will have to start again. Gradually whisk in 125ml milk and the wine. Cook for a minute or two, whisking constantly, then add a pinch of salt and the Gruyère. Whisk until the cheese has melted and the sauce is smooth and glossy – it should happily coat the back of a spoon without sliding off. Add another 1-2 tbsp milk until the right consistency is reached. Season with salt and ground white pepper.
Cut the scallops in half horizontally and put fleshy coins, side by side, in the deepest part of each scallop shell. Spoon over the Mornay sauce and put on a sturdy baking tray. Prop up each shell with a small scrumpling of foil so that the sauce stays level around each scallop.
Heat the grill to its hottest setting. Grill close to the red hot element for about 2 1/2 minutes or until the sauce is lightly browned and bubbling.