In a saucepan, combine the cornmeal, salt, and water. Gradually add the evaporated milk. Bring to a boil and stir until the mixture thickens. Remove from the heat, cover, and let cool.
Preheat the oven to 375°F and grease a shallow 9x13-inch baking pan.
Sauté the ground beef and onion in a skillet over medium heat until the onion softens and the beef is almost browned. Drain the excess fat.
Add the tomatoes, salt, pepper, and chili powder to the skillet. Stir and cook until the beef has finished browning.
Spread half of the cornmeal mixture in the prepared baking pan. Then add the meat mixture and top with the remaining cornmeal.
Bake for 35 minutes or until the top is golden brown.