Slice the potatoes into fairly large slices. Boil until done.
Heat the oil or butter and gently fry the sliced leek.
Preheat the oven to 350°F (180°C).
Melt the 2 Tbsp of butter in a small saucepan. Add in the flour. Cook for 2-3 minutes, stirring constantly. Remove from heat and stir in milk. Whisk vigorously until all lumps are gone. Return pan to heat and cook, whisking constantly, until thickened.
Remove your sauce from heat and stir in most of the cheese.
Place the sliced, cooked potatoes on the bottom of a baking dish.
Top with the leek, the peas and the bacon or ham.
Pour the sauce on top and sprinkle with the remaining cheese.
Bake for about 20 minutes, until golden.