Place the potatoes into a large sauce pan and add just enough cool water just until the tops of the potatoes are covered and they float slightly.
Place the pan over high heat and as soon as the water begins to boil, remove from the heat and drain the potatoes in a colander. ~5 minutes
While the potatoes are draining place the butter and oil in a 10″ Cast Iron Pan . When the butter begins to foam add in the yellow onions and gently stir to coat. Cook for 3-5 minutes or until softened.
Add in the potatoes and Montreal steak seasoning. Gently stir to coat the potatoes on all sides. Cook for 6-10 minutes or until the potatoes are browned on all sides.
When the potatoes are browned turn on the broiler.
Add the shredded pot roast to the potatoes and gently mix.
Top with the cheese and place the pan under the broiler for 30 seconds – 1 minute or until the cheese has melted.
Remove from the oven and garnish with the green onions.