For the chicken and marinade: Cut each chicken thigh into bite-size pieces, approximately 1 1/2 by 1 1/2 inches.
In a large resealable plastic bag, combine the fennel, mustard, salt and black pepper. Add the chicken and massage the bag so the chicken is thoroughly coated in the spices. Add the buttermilk and onions, and massage the bag again to mix the ingredients. Marinate the chicken in the refrigerator for at least 1 hour and up to overnight.
For the breading: Set up a breading station by lining up 3 small baking dishes or pie plates. In one dish, add the flour. In the second, whisk the eggs with 3 tablespoons water. In the third, mix together the panko, fennel, paprika, salt and black pepper.
A few pieces at a time, remove the chicken from the marinade and let the excess drip off before dredging in the flour. Shake off the excess flour and then dunk the chicken in the egg wash, letting the excess drip off. Finally, dredge in the panko mixture, making sure the chicken pieces are completely coated.
Place the breaded chicken on a wire rack set over a baking sheet and refrigerate, uncovered, for 15 to 20 minutes.
Fill a large, heavy skillet with about an inch of canola oil and heat to 350 degrees F over medium heat.
In batches, fry the chicken in the oil until golden brown all over, 3 to 4 minutes per side. Drain on a paper-towel-lined baking sheet.