Place mushrooms upside down (gills facing up) on a piece of parchment paper on a baking sheet and broil for about 6 – 8 minutes, until tender. Add a slice of cheese on each and broil for another minute or until the cheese is melted.

In a bowl, whisk the egg white powder and konjac flour together, and then add the Greek yogurt, lemon juice, white wine vinegar, and salt. Mix together until completely combined.

Add the tuna, celery, cucumber, and green onions and stir.

Remove the mushrooms from the oven, and transfer each mushroom onto a separate plate.

Pile tuna salad onto each mushroom.

Grind black pepper over each plate and serve.