Heat 2 tbsp of zsír (or pork fat) in a small pot until melted, add the garlic and peas, season well with a pinch of salt.
Simmer for 1-2 minutes (or until the frozen peas have defrosted), then add the water, cover and cook for 10 minutes or until the peas are cooked.
Meanwhile, in a separate pan, melt the remaining 2 tbsp zsír and then add the flour, and stir to make a roux. Add the milk little by little, stirring until incorporated into the roux. Continue until all the milk has been combined.
Add some of the pea cooking liquid to the sauce and whisk well until you achieve a thick but smooth sauce.
When the peas are cooked, add the sauce and the parsley to the peas, and bring to a simmer for a minute or two, then serve.