In a large skillet, heat a tablespoon of oil over medium heat. Add the ground beef, breaking it up with a spatula, and cook until browned, about 5-7 minutes. Add the chopped onion and sauté until translucent, roughly 3-4 minutes.

While the beef and onions are cooking, wash and slice the carrots and potatoes. Measure out the corn and green beans. Start layering with the sliced potatoes followed by the carrots, corn, and green beans in the slow cooker.

In a medium bowl, combine the can of mushroom soup with a splash of water to thin it out, stirring until smooth. Pour the mixture evenly over the layered vegetables.

Once the beef and onions are ready, gently fold them into the vegetable and sauce mixture in the slow cooker. Stir carefully to incorporate.

Cover the slow cooker with its lid, setting it to low and allowing the casserole to cook for 6-8 hours.

In the last 30 minutes of cooking, uncover the slow cooker and sprinkle a generous layer of sharp cheddar cheese over the top.