Thomas Keller's Croque Madame

Preheat the oven to 190C/370F. To make the mornay sauce, add the butter to a medium-sized saucepan and place over a low-medium heat. Add the chopped onion and a good pinch of salt, then gently stir until the butter is melted and the onions have softened. Next, add the flour, and gently stir until a roux (a thick paste) has formed. Cook, stirring, for one minute to cook out the flour but without browning the roux.

Add the milk and cream to the roux and whisk so that the roux has incorporated into the liquid. Pop in the bay leaf, peppercorns and cloves. Simmer the sauce gently, whisking regularly, for 30 minutes until the sauce is looking nice and smooth (the only lumps should be the onions!).

Once the sauce has cooked, strain it through a fine sieve into a clean saucepan to remove any lumps. Add nutmeg and white pepper and a little more salt. Transfer the grated cheese into the sauce and whisk until it's fully melted and looking lovely and smooth.

Meanwhile, lay out the bread slices and spread with butter. To assemble the sandwiches, turn over half the bread slices over so the unbuttered sides are facing up. Divide the ham slices among these four slices, making sure they don't extend over the edges of the bread. Place two slices of Swiss cheese on top of each half, then place the remaining slices of bread on top to finish, buttered side facing up.

Heat a large ovenproof frying pan over medium heat and transfer assembled sandwiches to the pan (you'll probably have to do this in batches). Cook until bread is golden and toasted, then flip and repeat on the other side. Transfer to the oven and cook for 2-3 minutes or until cheese is turning gooey. Once the cheese has melted, remove the sandwiches from the oven.

In the meantime, add vegetable oil to another frying pan and place pan over medium heat. Crack in eggs. Cook until the bottoms are set, then place the pan into the oven for 1 minute or until the top of the whites have just set. Remove from the oven.

Transfer sandwiches to serving plates and top with the fried eggs. Spoon the mornay sauce over the whites of the eggs, leaving the yolk uncovered. (Confession: You'll see in my video that I decided to go double-decker, and stack another sandwich on top at this point, but that's entirely your call!). To serve, season with salt and pepper and sprinkle with chives.