Prepare the grill for direct cooking over high heat (450° to 550°F). Brush the cooking grates clean. Grill the bell peppers over direct high heat, with the lid closed, until blackened and blistered all over, 15 to 20 minutes, turning occasionally.
Place the peppers in a medium bowl and cover with plastic wrap to trap the steam. Let stand for 10 to 15 minutes.
Cut away the stem and core from one pepper, then slit the pepper lengthwise, spread it open, and remove the seeds and white membranes. Peel away and discard the charred skin. Repeat with the remaining pepper.
Cut the peppers lengthwise into ⅓-inch-wide strips. In a bowl combine the pepper strips, oil, vinegar, rosemary, garlic, salt, and red pepper flakes. Cover and let stand at room temperature for 1 to 2 hours, or cover and refrigerate for up to 2 days (let stand at room temperature for 1 hour before serving).
Related Grill Skills
Just before grilling the bread, finely chop the basil and add it to the marinated peppers.
Prepare the grill for direct and indirect cooking over medium heat (350° to 450°F). Lightly brush both sides of the bread slices with the oil.
Brush the cooking grates clean. Toast the slices on one side over direct medium heat, with the lid closed, until golden brown on the first side, about 2 minutes. Transfer the slices, grilled side up, to a baking sheet.
Top the bread slices with the Fontina, dividing it evenly. Using a fork, remove the peppers from the marinade, letting the excess marinade drip back into the bowl, and arrange on the bread slices, dividing the peppers evenly.
Grill the bruschetta over indirect medium heat, with the lid closed, until the cheese is just melted, about 2 minutes. Transfer to a platter and serve right away.