Combine your beef, salt, pepper and onion granules in a bowl.
Take a handful of your beef and press it into a large circle cutter. Repeat with your remaining beef mix until you have 12 patties.
Heat a frying pan to high. Add a little oil and pop a couple of burgers in the pan. Fry for about 2 mins until the base is firm and caramelised, then flip and cook for a further 2 mins on the other side.
Melt your butter in a small saucepan. Add your flour and cook out for 2 mins until the roux is golden and starts to smell biscuity. Gradually add your milk in small increments, beating all the time to ensure there are no lumps before adding more milk. Bring your mixture to a boil, then simmer for 2 mins.
Add your parmesan, mustard, nutmeg, salt and pepper and stir to combine.
Lay the base of your buns on a large sheet pan side by side. Top each with a spoonful of béchamel, then your burger patties. Top each with a slice of ham, then a slice of cheese. Grill on a high heat for 2 mins until the cheese has melted.
Pop the top of your buns on your sliders and pour your remaining béchamel on top. Sprinkle with your grated Swiss cheese, then return to the grill for another 2 mins until the cheese is melted and the sauce is bubbling.