Line a greased 8-in. square baking dish with four bread slices. Layer with half of the tomatoes, cheese and onions. Top with remaining bread (slices will overlap). Layer with remaining tomatoes, cheese and onions.
In a small bowl, whisk the eggs, milk and salt. Pour over the top. Cover and refrigerate overnight.
Remove from the refrigerator 30 minutes before baking. Bake, uncovered, at 350° for 45-50 minutes or until a knife inserted in the center comes out clean. Let stand for 5 minutes before cutting.