Step 1: Preheat the Oven - Set your oven to 350°F before you start mixing anything. Giving the oven time to heat fully matters because you want the chicken to begin cooking evenly as soon as the casserole dish goes inside. While the oven warms, grab a large mixing bowl and a casserole dish big enough to hold all six thighs in one layer. Having everything ready now keeps the rest of the process calm, which I appreciate when dinner hour already feels a little noisy.
Step 2: Mix the Mustard Coating - Add the Dijon mustard, whole grain Dijon mustard, olive oil, minced garlic, salt, and black pepper to the large bowl. Stir everything together until the olive oil and seasonings are evenly worked into the mustard. You are looking for a thick, spoonable mixture with mustard seeds scattered throughout. Take a few extra seconds here to mix thoroughly because an even coating now means every piece of chicken gets the same tangy, garlicky flavor later.
Step 3: Coat the Chicken Thoroughly - Add the skinless chicken thighs to the bowl with the mustard mixture. Use tongs or clean hands to turn each thigh through the mixture until the chicken is coated on every side. Do not just spoon mustard over the top and call it done. Getting the coating around the edges and underneath the chicken helps the flavor stay with each bite instead of sitting only on the surface. If the thighs are different sizes, that is completely normal.
Step 4: Arrange the Chicken in the Dish - Transfer the coated chicken thighs to your casserole dish and arrange them in a single layer with the smoother side facing down. Give each thigh a little room instead of piling or overlapping the pieces. That space lets hot air move around the chicken and helps it roast instead of steaming. Scrape any useful mustard mixture toward one side of the bowl so you can use a little more after the chicken is flipped.
Step 5: Bake the First Side - Place the casserole dish in the preheated oven and bake the chicken at 350°F for 20 minutes. During this first stretch, the thighs begin releasing their juices and the mustard coating starts settling onto the meat. The kitchen will gradually pick up that warm garlic and mustard smell, which is usually the moment somebody in my house wanders in to ask when dinner will be ready.
Step 6: Flip and Brush the Chicken - After 20 minutes, carefully pull the casserole dish from the oven and place it on a heat safe surface. Use clean tongs to turn each chicken thigh so the smoother side is now facing up. Brush the tops with additional mustard mixture. For food safety, only use mixture that has not been saved for serving raw or that will continue cooking with the chicken. The goal is simply to refresh that coating so the top develops plenty of savory mustard flavor during the final bake.
Step 7: Finish Baking Until Juicy - Return the casserole dish to the oven and bake for about 25 minutes more. Chicken thigh sizes vary, so begin checking the thickest piece near the end of the cooking time. Insert an instant read thermometer into the thickest part without pressing it against the dish. The chicken is ready when it reaches 165°F. Once it is cooked through, the meat should look juicy and tender, with the mustard coating clinging to the surface.
Step 8: Garnish and Serve - Remove the casserole dish from the oven and let the chicken rest for a few minutes before serving. That short rest gives the juices inside the meat a chance to settle rather than immediately running onto the plate. Sprinkle fresh thyme over the top just before bringing it to the table. I like serving the chicken straight from the casserole dish on busy nights because those golden mustard coated thighs already look inviting without any extra fuss.