Step 1

Mix grainy mustard and honey together in a ceramic or glass bowl. Add chicken pieces and massage with marinade. Let sit to absorb flavors, about 30 minutes.

Step 2

Heat oil in a cast iron skillet over medium-high heat. Add onion and saute until soft and translucent, about 5 minutes. Add garlic and 2 tablespoons honey Dijon mustard. Season with salt and pepper. Reduce heat to medium and saute until onion and garlic are browned, 3 to 4 minutes. If necessary, add 1 tablespoon of water during this process.

Step 3

Add chicken to the skillet and remaining 2 tablespoons honey Dijon. Mix well and cover. Cook, stirring occasionally, until chicken is no longer pink in the centers, 7 to 10 minutes.

Step 4

Uncover, increase heat, and reduce sauce until creamy and thick, 3 to 5 more minutes.