Preheat the oven to 200 degrees C. Put the hash browns on a baking tray and cook for 30 minutes, until golden. Remove from the oven and leave the oven on.
Slice the bagels in half. Lay two pastrami slices over the bottom of each one, followed by two hash browns. Top with two slices of cheese and three pickle slices. Spread one teaspoon of English mustard (or more or less according to taste) over each of the top bagel slices. Put the bagel slices on top of the bagels to make a sandwich.
Wrap each bagel in foil. Bake in the oven for 5-7 minutes until warmed through and the cheese is melting. Enjoy immediately.