Heat a large skillet over medium heat. Add 1/2 teaspoon olive oil the skillet along with the mushrooms. Cook mushrooms for 2-3 minutes or until they begin to soften; season with salt and pepper. Remove mushrooms and set aside.
Add onion slices to the same skillet. Cook onions for 10-15 minutes, stir occasionally. You want the onions to turn caramel in color and be quite soft. When onions have caramelized, remove from heat.
Prepare the cheese sauce by stirring together butter and flour in a small saucepan over medium heat. Slowly whisk in the milk until the mixture is smooth. Stir in cheese.
To assemble the sandwich, place 2 slices of roast beef on two slices of toasted bread.
Top beef with cheese.
Divide the mushrooms and onions between the two sandwiches, top with remaining bread slices and serve.