For the tartar sauce:
1/2 cup mayonnaise (I used Miracle Whip)
1/2 cup nonfat plain Greek yogurt
1-1/4 tsp capers, drained
3 gherkin pickles
3/4 tsp dried dill weed
Juice of 1/2 lemon, or more to taste
1/2 tsp kosher salt
1/4 tsp fresh black pepper
4 4-oz pieces fresh tuna steak (or swordfish, or salmon)
1 Tbsp olive oil
Kosher salt and fresh black pepper
4 hamburger buns
Combine tartar sauce ingredients in a food processor, and pulse until the pickles and capers are chopped and everything is combined. Taste, and adjust seasoning with more lemon juice, salt and pepper, as needed. (Can be made hours or days ahead. Refrigerate until ready to use.)
Preheat grill, grill pan or broiler to high heat. Take the fish out of the refrigerator and allow it to come to room temperature while the grill is heating.
Cut the tuna to make four portions. Brush each piece with olive oil, and sprinkle on both sides with salt and pepper.
The fish should cook 10 minutes for each inch of thickness. If your fish is one inch thick, cook for 5 minutes per side. If it's half an inch thick, cook for 3 minutes on the first side and 2 minutes on the second side.