Place the mint, parsley, garlic and Serrano chiles in a food processor and process until coarsely chopped. Add the mustard and process until combined. With the motor running, slowly add 1/2 cup of the olive oil until emulsified, adding a bit more if needed.
Transfer the mixture to a bowl and whisk in a few tablespoons of cold water to thin to a sauce-like consistency. Season with salt, pepper and honey (if needed) to taste. Reserve.
For the spice rub, whisk together the paprikas, coriander, mustard, fennel, pepper and salt in a small bowl. Season the chicken breasts on both sides with salt and pepper. Rub the top side of the chicken breasts with some of the spice rub.