Prep the Vegetables: Finely shred the cabbage, ensuring that the pieces are all about the same size to ensure even cooking. Dice the onion and red bell pepper, and set them aside. Mince the garlic.
Sauté the Aromatics: Heat 1 teaspoon of olive oil in a large pan over medium heat. Once hot, add the minced garlic and diced onion. Sauté for about 2–3 minutes, stirring frequently, until the onion becomes soft and translucent.
Cook the Cabbage and Bell Pepper: Add the shredded cabbage and diced red bell pepper to the pan. Stir everything together, cooking for another 5–7 minutes. You want the cabbage to soften and begin to brown slightly, which adds a nice toasted flavor to the casserole. Stir occasionally to avoid burning, but allow the cabbage to cook long enough to develop a little bit of color.
Prepare the Soup Base: In a small bowl, mix the packet of cream of mushroom soup with ½ cup of water and ½ cup of skim milk. Stir until the soup mix is fully dissolved. This will serve as the creamy base of your casserole.
Assemble the Casserole: Preheat your oven to 350°F (175°C). In a large mixing bowl, combine the sautéed cabbage, bell pepper, corn kernels, and drained tuna. Pour the prepared soup mixture over the vegetables and tuna. Add 1 teaspoon of dried thyme, and season with salt and pepper to taste. Stir everything until well combined, ensuring the creamy soup mixture coats all the ingredients evenly.
Transfer to a Baking Dish: Grease a medium-sized baking dish lightly with a small amount of oil or cooking spray. Transfer the casserole mixture into the dish, spreading it out evenly.
Bake the Casserole: Bake in the preheated oven for 20–25 minutes, or until the casserole is heated through and the top is slightly golden and bubbly. If you prefer a crispier top, you can turn on the broiler for the last 3–5 minutes of cooking, but be sure to keep an eye on it to avoid burning.
Serve: Remove the casserole from the oven and let it cool for a few minutes before serving. If desired, you can sprinkle a bit of fresh thyme or a squeeze of lemon juice over the top for a fresh finish. Serve hot and enjoy!