Mashed Potato, Cheese, and Ham Pie

Ingredients
For the Filling:

800 g (about 4 medium) russet potatoes, peeled and chopped

60 g (4 Tbsp) unsalted butter

120 ml (½ cup) whole milk or cream

200 g cooked ham, diced

150 g sharp cheddar cheese, shredded

1 small onion, finely chopped

1 Tbsp olive oil

Instructions
Cook the Potatoes: