Step 1

Arrange a rack in the middle of the oven and place a rimmed baking sheet on the rack. Heat the oven to 400°F. Meanwhile, bring a large pot of heavily salted water to a boil. Remove any casings from 1 pound uncooked Italian sausage if needed. Finely dice 2 medium shallots (about 2/3 cup). Trim and cut the leaves from 1 pound broccoli rabe or baby broccoli crosswise into 1-inch pieces, then cut the stems into 1/3-inch pieces. If needed, grate 10 ounces mozzarella cheese on the large holes of a box grater (about 2 1/2 cups). Grate 2 ounces Parmesan cheese on the small holes of the box grater (about 1 cup).

Step 2

When the water is boiling, add 1 pound dried penne pasta and cook according to package directions until barely al dente, about 7 minutes (the pasta will not be cooked through). Reserve 1/3 cup of the pasta water, then drain the pasta.

Step 3

Heat 2 tablespoons of the olive oil in a large skillet over medium-high heat until shimmering. Add the sausage and cook, breaking the sausage into smaller pieces with a wooden spoon, until almost cooked through and beginning to brown, about 4 minutes.

Step 4

Add the shallots and cook, stirring often, until the shallots are soft and fragrant, about 2 minutes. Add all of the broccoli rabe pieces and 1/4 teaspoon kosher salt and cook, stirring often, until the broccoli rabe is just tender and leaves are wilted, about 5 minutes. Remove the pan from the heat. Taste and season the broccoli rabe with more kosher salt as needed.

Step 5

Place 4 cups marinara sauce, 1 large egg, and 1 teaspoon Calabrian chili paste if using in a large bowl and whisk to combine. Add the pasta, pasta water, sausage mixture, and 1 cup of the mozzarella, and stir to combine.

Step 6

Remove the baking sheet from the oven and brush the bottom and the sides with the remaining 1 tablespoon olive oil. Transfer the pasta mixture onto the baking sheet and spread into an even layer, leaving some tips of pasta poking up for extra-crispy tips. Sprinkle with the remaining 1 1/2 cups mozzarella and all of the Parmesan.

Step 7

Return the baking sheet to the oven and bake until the cheese is golden-brown and bubbly and the pasta is tender, about 25 minutes. Let cool for 10 minutes before cutting into squares and serving.