In a medium-sized bowl, mix all the ingredients until well combined.
Use a spatula to push butter to coat the pan. Place buns cut side down onto the buttered pan and cook for about 1 minute until golden and toasted. Turn around and toast the other side.
Assemble, place the lower part of the bun on a plate and top with the red pesto, sriracha, the fresh cabbage, and scoop a couple of tablespoons of your freshly made tuna salad. Add a slice of tomato and the sliced avocado and place the top part of the bun on top.