"For the best results, use the ripest in-season tomatoes you can find. Supermarket vine-ripened tomatoes will work, but the gratin won't be as flavorful as one made with locally grown tomatoes. Do not use plum tomatoes, which contain less juice than regular round tomatoes and will result in a dry gratin....the remainder [of the toasted bread scattered] over the top along with some Parmesan...create a crusty, savory topping that contrasted with the custardy interior."
" You can serve the gratin hot, warm, or at room temperature."
Adjust oven rack to middle position and heat oven to 350 degrees. Heat ¼ cup oil in 12-inch overnsafe skillet over medium-low heat until shimmering. Add bread and stir to coat. Cook, stirring constantly, until bread is browned and toasted, about 5 minutes. Transfer bread to bowl.
Return now-empty skillet to low heat and add remaining 2 tablespoons oil and garlic. Cook, stirring constantly, until garlic is golden at edges, 30 to 60 seconds, Add tomatoes, sugar, salt, and pepper and stir to combine. Increase heat to medium-high and cook, stirring occasionally, until tomatoes have started to break down and have released enough juice to be mostly submerged, 8 to 10 minutes.
Remove skillet from heat and gently stir in 3 cups bread until completely moistened and evenly distributed. Using spatula, press down on bread until completely submerged. Arrange remaining 1 cup bread evenly over surface, pressing to partially submerge. Sprinkle evenly with Parmesan.
Bake until top of gratin is deeply browned, tomatoes are bubbling, and juice has reduced, 40 to 45 minutes; after 30 minutes, run spatula around edge of skillet to looseb crust and release any juice underneath. (Gratin will appear loose and jiggle around outer edges but will thicken as it cools).
Remove skillet from oven and let stand for 15 minutes. Sprinkle gratin with basil and serve.