Heat the oven to 400 degrees F.
Coat the tomatoes in olive oil and season with salt and pepper. Roast 20 minutes to concentrate the flavor and burst the tomatoes.
Cut 2 large "planks" of bread each 1-inch thick by slicing the bread lengthwise horizontally. Reserve the rest of the loaf for another use.
When you remove the tomatoes from the oven, switch on broiler. Char bread on both sides then rub hot bread with cut garlic.
In a bowl, lightly mash the roasted tomatoes and combine with scallions and basil. Top the large planks with tomato mixture then cut each giant crostini into 4 pieces, 8 pieces total.