Preheat your oven to 350 degrees and spray an 8x13 casserole dish lightly with nonstick spray.
In a large bowl, combine the tuna, celery, red onion, relish, mayo, lemon juice, tomato, salt, and pepper. Set it aside.
Place the bottom half of your sliders into the bottom of your casserole dish.
Layer your ingredients over the bottom buns, starting with 6 slices of Colby Jack cheese, then the tuna mixture, then the remaining 6 slices of Colby Jack cheese, and finally place the bun tops on last.
In a small bowl, use your melted butter and brush it over the bun tops. Sprinkle garlic powder over the tops.
Bake in your preheated oven uncovered for 15-20 minutes or until the tops are toasted and the sliders are heated throughout, and the cheese is melted. Sprinkle with chopped chives after cooked.
Allow to cool for a few minutes. Slice the sliders and serve!