Start by getting a large, nonstick skillet on medium heat. Toss in the mushrooms, onions, jalapeno, and rosemary. Season with salt and pepper, and add in one tablespoon of the butter. Let this just cooke for a couple of minutes, then give it a good flip, or stir, whatever you are comfortable with. Lower the heat a bit, and continue to cook for about 20 minutes, stirring or flipping along the way. Your goal is to bring out those natural sugars and caramelize the mushrooms, and onions. The mixture will reduce in size as well. Once caramelized, set it aside.
Take the remaining butter and lightly butter all four slices of the bread. My skillet only holds two slices, so I laid them butter side down in a large, nonstick skillet on medium heat. Cook them until they are nice and toasted. Repeat with the remaining two slices.
Preheat your oven to 325 degrees.
Once toasted, lather the bottom stecca, or one slice of each bread with mayonnaise. Cover with two slices of swiss cheese, then top with the caramelized mushroom mixture. Top that with the two slices of harvarti cheese, then top with the other slice of bread.
Repeat with the other sandwich.
Place these onto a baking sheet and place into the oven for about 5-7 minutes, or until the cheese is nice and melted.
To plate, slice in half on the diagonal, and serve with a nice green salad.
Biting into this sandwich is freaking delicious. You get that wonderful earthiness from the caramelized mushrooms and the sweetness from the onions. Then boom, you get that flavor from the rosemary as a nice surprise. This was everything I was looking for when I came up with the idea, and that bread, well, don't even get me started on that. Those steccas are not only super easy to make, but probably the best sandwich bread I have had, and that's no lie.