1 tbsp olive oil 1 red onion, thinly sliced 1/8 tsp sea salt 1/4 tsp fresh ground black pepper 12oz pouched wild albacore tuna (about 1 1/2 cups) 2 stalks celery, finely chopped 2/3 cup 2% plain Greek yogurt 1 1/2 tsp mustard powder 1/2 tsp ground cayenne pepper, or to taste (TIP: If you'd rather skip the heat, use 1/2 tsp sweet paprika plus 1/2 tsp ground cumin instead.) 1/2 tsp dried or 1 1/2 tsp chopped fresh tarragon leaves 6 1 1/2-oz clices whole-grain bread 3 oz reduced-fat white or orange cheddar cheese, grated 3 scallions, thinly sliced

Preheat oven to 350 and line a large, rimmed baking sheet with foil.
In a large skillet on medium, heat oil. Add onion, salt and pepper and cook, stirring occasionally, until browned and tender, 10 to 12 minutes.
Meanwhile, to a medium bowl, add tuna, celery, yogurt, mustard powder, cayenne and tarragon; stir to combine.
Toast bread until golden brown and arrange on sheet. Spread one-sixth of tuna mixture over each bread slice. Top with onions and cheese, dividing evenly. Bake until cheese is melted, about 5 minutes. Sprinkle with scallions.