Preheat the oven to 450 degrees.
Trim off and discard the dried, tough part of the broccoli stem (about ½ inch). Working from the stem end, slice the broccoli lengthwise into 4 slabs.
Arrange the broccoli slabs in a single layer in a 9 × 13-inch baking dish. Drizzle with the olive oil, turning to coat, and season lightly with salt and pepper. Nestle the garlic slices among and under the broccoli slabs. Roast for 10 minutes. Flip and roast for 10 minutes more, until tender and browned.
Top the broccoli with the tomato-basil sauce and the mozzarella. Bake until the cheese is melted and bubbling, 5 to 8 minutes more.
Immediately garnish with the Parmesan and basil. Serve warm.