In a skillet, brown the ground beef with the diced onions and garlic until fully cooked. Drain any excess grease.
Spray the slow cooker insert with nonstick spray.
Layer half of the sliced potatoes in the bottom of the slow cooker.
Top with half of the cooked beef mixture.
Add half of the corn and half of the drained diced tomatoes.
Sprinkle with 3/4 cup of the shredded cheese.
Repeat the layers: potatoes, beef, corn, tomatoes, and the remaining cheese.
In a small bowl, whisk together the cream of mushroom soup and milk. Pour evenly over the layers in the slow cooker.
Cover and cook on LOW for 6-7 hours or on HIGH for 3-4 hours, until potatoes are tender.
Garnish with fresh parsley before serving, if desired.