Prepare the tuna salad: In a medium bowl, mix the tuna, Greek yogurt, Dijon mustard, olive oil, lemon juice, garlic powder, dill, salt, and pepper until well combined.

Assemble the melts: Preheat your oven broiler or toaster oven. Lay the bread slices on a baking sheet and spread a generous layer of the tuna mixture on each slice of bread. If using toppings like tomato slices or baby spinach, add them on top, then place a slice of cheese on top of each open-face sandwich.

Melt and toast: Place the baking sheet under the broiler for 3–5 minutes, or until the cheese is melted and bubbly. Keep an eye on it to avoid burning!

Serve: Remove from the oven, let cool slightly, and serve immediately. Pair with a side of mixed greens or a cup of soup for a balanced meal.