Arrange a rack in the upper third of the oven and heat the oven to 400°F. Coat an 8-inch square or 2-quart baking dish with unsalted butter and place on a baking sheet.

Cut 1 ½ pounds broccoli crowns or florets into 1-inch pieces, and peel and cut any large pieces of stalk crosswise into ½-inch-thick pieces (about 9 cups total).

Dice 1 small yellow onion (about ¾ cup). If needed, grate 4 ounces low-moisture mozzarella cheese on the large holes of a box grater, or measure out 1 cup store-bought shredded. Finely grate ½ ounce Parmesan cheese on the small holes of a box grater (about 2 tablespoons), or measure out 2 tablespoons store-bought grated.

Melt 1 tablespoon unsalted butter in a large high-sided skillet or Dutch oven over medium heat. Add the onion, 1 teaspoon Italian seasoning, ½ teaspoon black pepper, and ¼ teaspoon kosher salt. Cook over medium heat, stirring occasionally, until the onion is softened and translucent, about 5 minutes.

Add 1 (about 16-ounce) jar Alfredo sauce, ½ cup milk or water, and half of the mozzarella cheese. Bring to a simmer, stirring frequently until the cheese is melted. Taste and season with more kosher salt as needed.

Remove the skillet from the heat. Add the broccoli and stir until well-coated in the sauce. Transfer to the baking dish and spread into an even layer. Sprinkle with the remaining mozzarella and the Parmesan.

Bake on the baking sheet (to catch any drips) until the broccoli is tender and the sauce is bubbling, about 30 minutes. Let cool for 10 minutes to let the sauce thicken before serving.