Preheat oven to 375°F or preheat the grill to medium-high (about 400°F). Bring a medium-sized pot of water to a boil.

Cube 2 pounds sirloin steak into bite-sized pieces. Trim excess fat if desired. Set aside.

Cube 2 pounds red potatoes into bite-sized pieces. Parboil for 5 minutes or microwave with 3 tablespoons water for 3 minutes. Potatoes should be still firm but starting to tender.

Lay out 4 pieces of aluminum foil, approximately 14x18 inches each. Distribute 1/2 pound of steak, 1/2 pound of potatoes, and about 1 tablespoon of minced jalapenos onto each piece. Adjust jalapenos to taste.

In a small bowl, mash together 6 tablespoons softened salted butter, 2 tablespoons minced parsley, 2 teaspoons salt, 2 teaspoons black pepper, 2 teaspoons garlic, and 1 tablespoon Worcestershire sauce. Divide and spoon over the ingredients in each packet. Top each with a sprig of rosemary.

Fold the foil over to seal tightly, ensuring no leaks.

Bake in the oven for 25 to 30 minutes or grill over medium-high heat for 15 to 20 minutes, flipping as needed. Cook longer if desired for more doneness.