On medium heat, place cast iron skillet on the stove and drizzle avocado oil over the bottom of the pan. Let warm up. Prepare chicken while the skillet is heating up. Cut fat, veins, or anything else off of each chicken breast you don't like having on there. Then cube into approximately 3oz sections. Pat each piece dry and season with McCormick's Grill Mate Roasted Garlic & Herb seasoning.
Turn the heat under the skillet down to medium-low. Place the pieces of chicken into the preheated skillet. You are not cooking the chicken through on the stove. You are just braising each side. Only cook the chicken until the outside on each piece is light brown. Depending on how large your skillet is, you may need to do this in batches.
Once you are done braising each chicken piece, put all of the chicken back into the skillet, cover with foil, and place in a preheated 350°F oven. Cook for 20 minutes, or until the center of the chicken is no longer pink.
Take the skillet out of the oven and place back on top of the stove. Take the chicken out and place on a plate, cover with the foil to keep the heat in.
Turn the heat back on under the skillet to medium and add in the mayonnaise, both mustards, both honeys, and if you have any left over of the McCormick Grill Mate seasoning add that in as well. Whisk to combine and begin pouring the chicken stock into the mustard mixture. Whisk continually until the consistency thickens up. This should only take about 5 minutes. Once the mustard sauce is finished, pour it over the cooked chicken. Top with chopped chives and serve immediately.