Preheat grill to medium-high. Using 2 tablespoons of the olive oil, brush both sides of the baguette slices and grill until golden brown and lightly charred around the edges, about 1 to 2 minutes on each side. Remove from grill and set aside. Using another 2 tablespoons olive oil, brush eggplant and onion and grill until lightly charred, 3 to 4 minutes on each side. Remove eggplant and onion from heat, season with salt, and set aside, on a cutting board, until cool enough to handle. Slice both into 1/2-inch cubes. Combine eggplant, onion, tomato, garlic, remaining tablespoon olive oil, black pepper, pepperoncini and pickling juice in a bowl. Toss to combine and check for seasoning, adding additional salt, if necessary. Assemble toasts on a serving platter and place a heaping spoonful of vegetable mixture on top of each. Sprinkle with cheese and serve.