To make the citrus slaw: To a large bowl, add the cabbage, 1 tablespoon of the sugar and 1/2 teaspoon of the salt. Toss, cover and refrigerate for 30 minutes to brine. Toss in the parsley right before adding the dressing.
Into a jar, add the mayonnaise, Dijon, poppy seeds, lemon juice, lime juice, the remaining 1/2 tablespoon sugar, 1/2 teaspoon salt and lots of freshly cracked pepper. Close the jar and shake until well mixed. Pour the dressing onto the slaw and toss to combine.
To make the tuna-cakes sliders: Heat the olive oil in a pan over medium heat. Throw in the celery, chile, garlic, cayenne, celery seeds, salt and freshly cracked pepper and saute for 3 to 4 minutes. Remove from the heat and cool.
In a big bowl, add the tuna, breadcrumbs, parsley, cream, Dijon, green onions and egg, and mix. Add the cooled garlic-chile mixture and mix again.
Lightly flour your hands and shape the tuna mixture into small patties. Dredge them in flour and then place in the freezer for 10 minutes to firm up.
Heat the canola oil to 350 degrees F in a large frying pan. Fry until crispy and heated through, 5 to 7 minutes. Drain.
Place a patty onto a slider bun and top with some of the slaw.