Set a small pot half filled with water on to boil then turn down to a simmer. Place the four yolks and vinegar into a stainless steel bowl. Place onto the pot of simmering water whisking until the mixture is very thick. Lift the bowl every now and then if your mixture looks like it might scramble. Remove from heat and very slowly drizzle in the butter, whisking all the time. Get someone to hold the bowl while you whisk. Finish sauce with a good squeeze of lemon juice and a little sea salt. Put into a small container with a lid on, and put aside.
Place the toasted muffin onto warm serving plate. Top with a small handful of fresh baby spinach. Place three slices of warm fried haloumi over.
Spread 1 Tbsp of pesto over the cheese. Use store-bought pesto or make your own with basil leaves, garlic, ground almonds, parmesan, extra virgin olive oil, salt and pepper, whizz all in a food processor and check for seasoning.