Put the orange juice, lemon juice, olive oil, dijon mustard, kosher salt, pepper, and crushed red pepper flakes in a mason jar or container with a lid. Shake until combined.
Trim the chicken thighs if needed and place them in a gallon size plastic bag. Add half of the citrus marinade and more kosher salt, pepper, and crushed red pepper flakes. Massage the bag until the chicken is coated and marinate in the fridge several hours or overnight.
Preheat your grill or Blackstone griddle to medium. Once hot, add the chicken (cooking oil first if using a griddle) and cook 5 to 6 minutes per side.
Serve with the other half of the citrus marinade for dipping.Note: The citrus marinade is thin, if you want a creamier or thicker dipping sauce mix it with a little mayo.