Trim about 1 inch off broccoli stems and discard. Slice the broccoli stalks in half lengthwise to have 4 pieces total.
Place the broccoli and the garlic cloves on a 9-by-13-inch baking pan and drizzle both sides of the broccoli florets and stems with olive oil. Season both sides with salt and roast cut side up about 10 minutes, until the broccoli is browned on the bottom. Turn the broccoli and garlic and roast an additional 10 minutes until tender and browned and crisp.
Top with marinara sauce and mozzarella. Return to the oven and bake until hot and the cheese is melted, about 10 minutes.