Adjust rack to top position and preheat oven to 425 degrees. Bring a large pot of salted water to a boil. Wash and dry all produce. Core, deseed, and thinly slice bell pepper.

Toss bell pepper on one side of a baking sheet with a drizzle of olive oil and a pinch of salt and pepper. Add sausage to opposite side of sheet and drizzle with olive oil. Roast on top rack until bell pepper is browned and tender and sausage is cooked through, 20-25 minutes.

Once water is boiling, add pasta to pot. Cook until al dente, 8-10 minutes. Reserve 1 cup pasta cooking water (2 cups for 4 servings), then drain.

Melt 2 TBSP butter (3 TBSP for 4 servings) in pot used for pasta over medium-high heat. Stir in flour and garlic powder and cook, stirring, for 1 minute. Slowly whisk in milk, 1/2 cup reserved pasta cooking water (1 cup for 4), and stock concentrate until combined. Bring mixture to a boil, then remove from heat. Stir in cheese until melted. Season generously with salt and pepper.

Once sausage is done, remove from oven and let cool slightly. Transfer to a cutting board and slice on a diagonal into 1/2-inch-thick pieces. Add to pot with sauce along with bell pepper and pasta. Stir, adding splashes of reserved pasta cooking water as needed, until creamy. Season generously with salt and pepper. Transfer mixture to an 8-by-8- inch baking dish (for 4 servings, use a 9-by-13-inch dish).

Preheat broiler to high. In a small bowl, combine panko and a drizzle of olive oil. Season with salt and pepper, then sprinkle over pasta mixture. Broil on top rack until panko is golden brown, 2-3 minutes. (TIP: Watch carefully for any burning.) Drizzle with as much hot sauce as you like and serve.