Preheat the oven to 350 degrees . Place the garlic and lemon juice in a bowl and whisk in 3 tablespoons EVOO; season with lots of pepper.
Heat the remaining 1 tablespoon EVOO, 1 turn of the pan, in a medium skillet over medium-high heat. Add the sliced onion and fennel bulb and cook to crisp-tender, 6 to 7 minutes; season with salt and pepper.
Add the tuna to the garlic-lemon dressing. Combine with the finely chopped onion, the capers, olives, parsley and fennel fronds; toss to combine. Divide the tuna among the bread slices and top with the sauteed fennel and onions, then the fontina or provolone. Pop into the oven and bake until the bread is crisp, the tuna is warm and the cheese is melted and lightly browned, about 15 minutes.