Prepare the Marinade - In a mixing bowl, combine the Swedish mustard, cumin, smoked paprika, garlic, lime juice, and red onion. Mix until well combined. - Make sure to finely mince the garlic and red onion so that they blend well with the other ingredients in the marinade.
Marinate the Chicken - Add the boneless chicken thighs to the marinade and refrigerate for at least 30 minutes or up to 12 hours. - For best results, marinate the chicken overnight to allow the flavors to fully develop. If you are short on time, marinate the chicken for at least 30 minutes before grilling.
Prepare the Grill - Preheat your grill to medium-high heat. Use a grill brush to clean the grates and lightly oil them to prevent sticking. - If you don't have a grill, you can use a grill pan or a cast-iron skillet to cook the chicken instead.
Grill the Chicken - Remove the chicken from the marinade and discard the remaining marinade. Grill the chicken for 6-8 minutes on each side, or until cooked through and slightly charred on the outside. - Use kitchen tongs to flip the chicken thighs on the grill and avoid piercing them with a fork, which would let the juices escape and make them dry. Also, for a perfectly cooked chicken make sure to let it rest for 5-10 minutes before slicing it.
Grill the Pineapple - While the chicken is grilling, add the pineapple slices to the grill and cook for 2-3 minutes on each side, or until slightly charred. - Make sure to slice the pineapple into thick rings so that they don't fall apart on the grill. You can also use canned pineapple rings if fresh pineapple is not available.
Slice and Serve - Once the chicken and pineapple are grilled, remove them from the grill and slice the chicken into thin strips. Serve the chicken and pineapple on warm corn tortillas and top with fresh cilantro. - To warm the tortillas, wrap them in plastic wrap and microwave for 30 seconds or heat them directly on the grill for a few seconds on each side. You can also serve the chicken and pineapple on lettuce cups for a low-carb option.