Bring a large pot of water to a rolling boil over high heat. Add the kosher salt and jumbo pasta shells. Cook according to the package directions, stopping approximately 1–2 minutes before the recommended cooking time so the shells remain al dente. Drain carefully and rinse briefly under cool running water to stop the cooking process. Arrange the shells open-side down on a clean baking sheet so they do not stick together.

Pat the thinly sliced sirloin dry with paper towels. Season it with the kosher salt, black pepper, and garlic powder listed for the filling. If slicing the beef yourself, place it in the freezer for approximately 15–20 minutes beforehand to firm it slightly. This makes it easier to cut into thin strips.

Heat the olive oil in a large 12-inch skillet over medium-high heat. Add the beef in two batches if necessary, keeping the pieces in a relatively even layer. Cook for approximately 2–3 minutes per batch, stirring or turning until browned and cooked through. For food safety, cook the sirloin to at least 145°F (63°C), followed by a 3-minute rest. Transfer the beef to a plate and set aside.

In the same skillet, add the sliced onion, green bell pepper, and red bell pepper. Cook over medium heat for 5–6 minutes, stirring occasionally, until the vegetables begin to soften. Add the sliced mushrooms and cook for another 5–6 minutes, until they release most of their moisture and begin to develop golden edges. The vegetables should be tender but not mushy, with very little excess liquid remaining in the skillet.

Add the minced garlic to the vegetables and cook for approximately 30 seconds, until fragrant. Return the cooked beef and its accumulated juices to the skillet. Add the Worcestershire sauce and stir until the beef and vegetables are evenly coated. Cook for another minute, then remove the skillet from the heat. If the beef slices are particularly long, chop the filling into smaller, bite-sized pieces so it fits neatly inside the pasta shells.

Preheat the oven to 375°F (190°C). Melt the butter in a medium saucepan over medium heat. Add the flour and whisk continuously for approximately 1–2 minutes. The mixture should form a smooth paste and develop a lightly nutty aroma without turning dark brown. Slowly pour in the warmed milk and beef broth, whisking constantly to prevent lumps.

Continue cooking the sauce over medium heat for approximately 4–6 minutes, stirring frequently. It should begin to bubble gently and thicken enough to coat the back of a spoon. If lumps appear, whisk vigorously until smooth. Avoid rapidly boiling the sauce, which can cause the milk to scorch along the bottom of the saucepan.

Reduce the heat to low: Add the 1½ cups of shredded provolone cheese in small handfuls, stirring until each addition has melted before adding more. Season with the kosher salt and black pepper listed for the sauce. The finished sauce should be smooth, creamy, and pourable. If it becomes excessively thick, stir in a small splash of warm milk. Remove the saucepan from the heat.

Spread approximately ¾ cup of the provolone sauce evenly over the bottom of a 9 × 13-inch baking dish. This prevents the pasta shells from sticking and helps keep their bottoms moist during baking. Arrange the cooked shells nearby with the open sides facing upward, ready for filling.

Spoon approximately 2 tablespoons of the beef and vegetable filling into each cooked pasta shell, dividing the mixture as evenly as possible. Arrange the stuffed shells in the baking dish, filling-side up. Place them close enough together to remain upright, but avoid stacking or crushing them.

Spoon the remaining provolone sauce evenly over and around the stuffed pasta shells. Sprinkle the mozzarella and remaining provolone cheese over the top. Make sure the edges of the pasta are lightly coated with sauce so they do not become excessively dry while baking.

Cover the baking dish tightly with aluminum foil, taking care to keep the foil from touching the cheese if possible. Bake at 375°F (190°C) for 20 minutes. Carefully remove the foil and continue baking for another 10–15 minutes, until the cheese is completely melted, the sauce is bubbling around the edges, and the casserole is heated throughout. The center should reach 165°F (74°C). For additional browning, broil for 1–2 minutes at the end, watching closely to prevent the cheese from burning.

Remove the baking dish from the oven and allow the stuffed shells to rest for 10 minutes. This gives the cheese sauce time to settle and makes the shells easier to serve without the filling spilling out. Sprinkle with the optional chopped parsley just before serving.

Divide the stuffed shells among six plates, allowing approximately four shells per serving. Spoon some of the creamy provolone sauce from the baking dish over each portion. Serve warm with a crisp green salad, roasted vegetables, or garlic bread.