Preheat oven to 350 degrees. Pare eggplant, cut into 1" cubes. Boil with onions and 2 t. salt in 1" boiling water until tender, about 5 minutes. Spread half of the crumbs in greased 8" square baking dish. Arrange eggplant and onions evenly on top. Sprinkle with salt and pepper. Top with rest of cracker crumbs. Pour milk into corners of pan to cover bottom. Pour butter over top. Bake uncovered for 45 minutes.