Preheat: Panini grill to high setting or skillet to moderate heat
Brush onion with olive oil and set aside. In a medium bowl, toss eggplant and zucchini with olive oil to coat. Grill vegetables until just tender, about 5 minutes.
Butter one side of each slice of bread (or brush with olive oil) and place the bread slices buttered side down. Layer half of the Fontina cheese, zucchini, roasted red peppers, eggplant, and red onions, and top with the remaining Fontina cheese folding back the edges of the cheese if they extend past the edges of the bread. Place the top slice of bread over the cheese, buttered side up.
Grill the sandwiches in a moderately hot skillet or in a Panini grill for about 8 minutes until the bread is golden brown, the cheese has melted, and the vegetables are heated through. If working in a skillet, turn sandwiches over halfway through grilling. Cut each in half on the diagonal and serve.
Serves: 4