Prepare the Jalapeños:

1. Preheat the oven to 425°F (220°C).
2. Place the jalapeños on an aluminum-lined baking sheet.
3. Roast for 15-20 minutes, turning halfway through until they develop some color and are tender.
4. Reduce oven temperature to 350°F (175°C) and grease a medium-sized (3-quart) casserole dish.

Cook the Pasta:

1. Cook the elbow macaroni according to package directions, but reduce the cooking time by one minute.
2. Drain the pasta and set aside.

Prepare the Sauce:

1. In the same pot used for the pasta, melt 4 tablespoons of butter over medium heat.
2. Whisk in the flour and cook for 1 minute, stirring constantly.
3. Gradually add the milk, whisking frequently.
4. Bring the milk mixture to a boil, then remove from heat.
5. Stir in the dry mustard, sharp cheddar cheese, Gruyere cheese, roasted and minced jalapeños, salt, and pepper.
6. Mix until all the cheese is melted and the sauce is smooth.

Assemble the Dish:

1. Add the cooked macaroni to the cheese sauce, stirring until well coated.
2. Pour the mixture into the prepared casserole dish.

Prepare the Topping:

1. In a small bowl, combine the Panko bread crumbs, grated Parmesan cheese, and 1 tablespoon melted butter.
2. Sprinkle the topping evenly over the mac and cheese.
3. Optionally, slice a fresh jalapeño and place the slices on top of the bread crumbs. Dot with butter.

Bake:

1. Bake at 350°F (175°C) for 20 minutes, or until the topping is golden brown and the mac and cheese is bubbly.