Preheat the oven to 400°F. Drizzle 1 tablespoon olive oil over a large rimmed baking sheet.
Keeping the stem intact, cut the cauliflower into 1-inch thick slices. Arrange the “steaks” on the baking sheet, adding any loose florets to the pan as well. Drizzle the cauliflower with 1 tablespoon olive oil and season very lightly with salt and pepper. Roast in the oven until undersides are golden brown, about 15 minutes. Using a spatula, carefully flip the cauliflower and return to the oven and roast until tender, about 10 minutes more.
While cauliflower roasts, make the vinaigrette: Combine the garlic and 3 tablespoons olive oil in a small skillet over moderately high heat until garlic begins to brown. Remove from the heat and whisk in lemon juice, capers, ¼ teaspoon each salt and pepper and remaining 2 tablespoons olive oil. Cover to keep warm.
When cauliflower is tender, sprinkle the shredded fontina over it and return it to the oven and roast until melted and bubbling, about 5 minutes. Remove from the oven and transfer to a serving platter. Stir the parsley into the vinaigrette and spoon over the cauliflower. Serve at once.