Preheat a grill to medium-high, and brush grates with safflower oil. Soak 4 pieces of kitchen twine in water. Separate scallions into 4 smaller bunches; secure with twine. Brush scallions, tomatoes, and peppers with olive oil, and season generously with salt and pepper. Sprinkle cut sides of tomatoes with sugar.
Grill vegetables, turning occasionally, until tender and well charred, 5 to 6 minutes for scallions and tomatoes and about 10 minutes for peppers.