Take the sliced deboned pork loin out of the fridge 30 minutes before cooking. Season the pork with salt & pepper on both sides.
In a frying pan or shallow cast iron skillet add olive oil on medium-high heat. Let the pan heat up for a few minutes.
Once hot, cook the pork loin fillets for 2-3 minutes on each side. After turning, add 1 tsp butter to the pan.
Once pork fillets are done searing, transfer them to a plate. Don't overcrowd the pan, sear 3-4 fillets at once.
Deglaze the pan with a splash of water if needed. Reserve the juices from the bottom of the pan.
Add 1 tbsp olive oil to the same pan and add mushrooms, cook for 2-3 minutes before adding leeks.
Add garlic and cook the veggies for about 5-6 minutes until they've released their water and it has mostly cooked off.
Sprinkle flour or cornflour over and cook for about 1 minute, stirring constantly.
Pour in white wine, milk, water or stock. Stir well so the flour dissolves and let it cook for 1-2 minutes.
Stir in the cream and mustard.
Add spinach, followed by pork and all its juices. Cover with a lid and cook for 2-3 minutes until pork is warmed through.
Season the sauce with salt & pepper if needed, then serve immediately with creamy mash, rice, or gnocchi.
