Preheat oven to 350ºF. Butter a 13 × 9 × 2-inch baking dish; set aside. Use tomatoes held at room temperature until fully ripe. Dice tomatoes (makes about 3 cups); place in a large bowl; set aside. Place bread on cookie sheets in a single layer; bake until toasted, about 5 minutes, turning occasionally. To the bowl with the tomatoes, add toasted bread cubes, Monterey Jack cheese, Parmesan cheese, onion and Italian seasoning. Spoon into prepared baking dish. In a large bowl, combine milk, eggs and salt; pour over tomato/bread mixture. With a spatula, press bread down to cover with milk mixture. Let stand for 5 minutes. Bake at 350 until a knife inserted in the center comes out clean, 30 to 40 minutes. Cool slightly before serving.