Spread tomato slices on paper towel-lined baking sheet. Sprinkle both sides with coarse salt. Let drain slightly, about 15 minutes.
Heat oven to 375 degrees F. Coat 2-quart oval baking dish with nonstick cooking spray. In medium-size bowl, stir together ricotta cheese, basil, egg yolks, flour and onion salt. Stir in half the grated cheese, about 1/2 cup.
In food processor, process English muffins until coarse crumbs are formed (about 1-1/4 cups). Sprinkle 1/2 cup crumbs over bottom of prepared baking dish.
Place single layer of tomato slices, including the ends, on top of crumbs. Carefully spread all the ricotta cheese mixture over tomatoes. Sprinkle with 2 tablespoons of English muffin crumbs. Top with remaining tomato slices, slightly overlapping. Brush with olive oil.
Bake gratin in 375 degree F oven for 30 minutes. Sprinkle with remaining English muffin crumbs and remaining 1/2 cup grated cheese. Bake until cheese is melted and topping is lightly browned, about 15 minutes. Let stand 5 minutes before serving. Makes 6 servings.