Preheat the grill to high heat.
Brush the bread with olive oil and sprinkle with salt and pepper, to taste. Put on the grill and cook until lightly toasted. Brush the tomatoes with canola oil and grill until slightly charred. Remove and coarsely chop. Remove the bread from the grill and spread the slices with mascarpone. Season with salt and pepper, to taste. Put the bread back on the grill over indirect heat to warm the cheese. Remove the crostini from the grill to a serving platter and top with the tomato and scallions. Drizzle with balsamic glaze, sprinkle with more salt and pepper, to taste, and serve.