Open the two cans of tuna and drain them thoroughly, pressing gently with the lid or using a fine-mesh strainer to remove as much excess liquid as possible. Properly draining the tuna is an important step because it prevents the filling from becoming watery and helps keep the bread from getting soggy. Transfer the drained tuna to a medium mixing bowl and use a fork to gently break it apart into smaller, evenly sized flakes for a smoother, more consistent filling.

Add 2 tablespoons of Greek yogurt and 1 teaspoon of Dijon mustard to the bowl with the tuna. Stir everything together thoroughly until the mixture becomes creamy and well combined. Make sure the Greek yogurt and mustard are evenly distributed so every bite has balanced flavor and texture. If desired, taste the mixture and adjust with a small pinch of black pepper or additional mustard according to your preference.

Arrange the 4 slices of whole grain bread on a baking sheet lined with parchment paper for easier cleanup. Divide the prepared tuna mixture evenly between two slices of bread, spreading it all the way to the edges using the back of a spoon or spatula. Even distribution ensures that every bite of the tuna melt contains the perfect amount of flavorful filling.

Place 1 slice of cheddar cheese over each prepared tuna-topped bread slice. Position the cheese so it covers most of the tuna mixture, allowing it to melt evenly across the surface during cooking. This step creates the classic gooey, cheesy layer that makes tuna melts so comforting and satisfying.

Transfer the baking sheet to the oven and place it under the broiler. Broil the tuna melts for 2–3 minutes, watching them very closely throughout the entire cooking process. The cheese should melt completely, become bubbly, and develop a few light golden spots on top. Since broilers work quickly, avoid stepping away from the oven to prevent the cheese from burning.

Carefully remove the baking sheet from the oven using oven mitts or heat-resistant gloves. Allow the tuna melts to rest for 1–2 minutes before serving. This brief cooling period allows the filling to settle slightly, making the sandwiches easier to handle while also reducing the risk of burning your mouth on the hot cheese.

Serve the tuna melts immediately while the cheese is still warm, creamy, and perfectly melted. Enjoy them on their own or pair them with a fresh side salad, crunchy vegetables, fresh fruit, or a comforting bowl of soup to create a complete and satisfying high-protein meal that is both nourishing and delicious.