Spread butter on one side of each slice of rustic oven sourdough bread.
Toast the buttered side of the bread until it turns golden and crispy.
In a bowl, mix together the drained tuna, chopped celery, chopped green onion, diced dill pickle, Greek yogurt, Dijon mustard, garlic powder, lemon juice, salt, and pepper to create the tuna mixture.
Flip the toasted bread slices over and place low fat cheddar cheese on top of each slice.
Add the tuna mixture on top of the cheese.
Return the bread to the toaster or oven until the cheese is melted and the topping is crispy.
Serve hot.
