Preheat the oven to 375 degrees F.
Crumble the poached tuna into a medium mixing bowl. Add the onions, eggs, garlic, and capers. Mix thoroughly. Season the mixture with salt and pepper. Stir in enough mayonnaise to moisten the salad, about 1/2 cup or according to preference.
Spread 1 to 2 tablespoons of mayonnaise over 6 slices of bread. Top each slice with a generous 1/3 cup of the tuna mixture. Top each tuna salad with a tomato slice and a slice of cheese. Place cheese side up on a baking sheet and bake for 3 to 4 minutes or just until the cheese is melted. Top each sandwich with 3 to 4 lightly salted avocado slices and serve open-faced or top with the remaining pieces of bread.
Cut sandwiches in half diagonally and serve with crispy potato chips and dill pickle spears.
Yield: 6 sandwiches