Roast Beef and Smoked Gouda Sandwich

This is my hands-down FAVORITE sandwich. It's super simple to prepare and is absolutely delicious. Smoked gouda is one of the best cheeses I've ever eaten. If you haven't tried it, you don't know what you've been missing! It's smoky and creamy and pairs excellently with roast beef. This sandwich is one of those things I make when I am too tired to make a more elaborate meal. It's an easy sandwich that is great for lunch or a summer supper.
Ingredients:
2 sub rolls or a baguette
6-8 thin slices of deli roast beef
four slices of smoked gouda (make sure it's smoked and not plain!)
6 button or baby bella mushrooms, sliced
1 tsp. butter or olive oil
salt and pepper
2 tbsp. mayonnaise
1 tbsp. creamy horseradish (adjust based on how much 'kick' you want)

Assembly:
Split sub rolls. In a saute pan, add oil/butter and cook mushrooms over medium-high heat until golden. Add salt and pepper to taste. Add roast beef to the sub rolls. Next, add mushrooms and cover with 2 slices of cheese per sandwich. Toast in a 350 degree oven (open-faced) 5-7 minutes or until cheese melts. While sandwiches toast, mix horseradish and mayonnaise. Remove from oven and spread horseradish/mayonnaise sauce inside the bun. Press sandwich together and serve.