Vegan Cheese & Onion Bakes (better than Greggs)

Name: Vegan Cheese & Onion Bakes (better than Greggs)
Prep time: 15 min
Cook time: 40 min
Total time: 55 min
Cuisine: British
Ingredients: 400 g potatoes ((peeled and cut into chunks)); 1 pinch salt & black pepper; 1 tablespoon olive oil; ½ large onion ((halved, peeled and thinly sliced)); ½ large leek ((washed well and thinly sliced)); 1 clove garlic ((peeled and crushed or finely grated)); 150 g vegan cheddar ((grated)); 2 tablespoons nutritional yeast ((also called nooch, optional, but adds flavour)); 2 teaspoons dijon mustard ((alternatively English mustard or yellow American mustard)); ¼ teaspoon white pepper ((more if you like)); 640 g puff pastry ((2 x 320g/11⅓ oz ready rolled pastry, if your pastry is a little bigger, that is fine too)); 2 tablespoons olive oil ((to brush the pastry before baking))