MAKE THE ROASTED VEGETABLES: Mix the olive oil, garlic, salt, and pepper together in a small bowl and set aside.
Stand the bell pepper up vertically and slice from the stem end downward on each of the 4 sides of the pepper. You will have four flat pieces of pepper. Cut the zucchini in half lengthwise, and then slice the halves again vertically so that you have 4 long pieces of zucchini. Slice the onion into 4 pieces horizontally, as in cutting for onion rings.
Place the vegetables in one layer in a small roasting pan and drizzle them with the olive oil mixture. Toss vegetables so that they are well coated with the oil. Transfer to the oven and roast for 20 minutes. Remove the pan from the oven, turn the vegetables over, return to the oven, and continue roasting for 10 to 15 minutes longer, until the vegetables are golden brown. Remove the pan from the oven and set aside.
When the vegetables are cool enough to handle, use a sharp knife to cut the bell pepper and zucchini into strips 1/4 inch wide and separate the onion into rings. Set aside.
MAKE THE CROSTINI: Position the oven rack so that it sits about 3 inches from the heat source. Preheat broiler to high.
Cut the baguette at a diagonal angle into 1/3-inch-thick slices. Using a pastry brush, sparingly coat both sides of the bread with olive oil and place the bread on a baking sheet. Transfer to the oven and toast, turning once, until the bread is a light golden brown on both sides. (Alternatively, place the slices of bread in a panini grill and toast until the bread is light golden brown and crisp.).
Remove the toasts from the oven and rub each toast lightly on one side with the whole garlic clove. Turn the oven to 350°F (175°C).
Return the toasts to the baking sheet. Top each piece of toast with slices of smoked mozzarella to cover the surface of the bread. Arrange the vegetables over the cheese, with the zucchini at one end of the slice of bread, the red bell pepper in the center, and the onion at the other end.