Preheat your oven to 200°C (400°F). As the oven warms, take your chicken breasts out of the fridge and let them sit on the counter for about 10 minutes — this helps them cook more evenly.

Make the glaze: In a medium mixing bowl, whisk together the honey, whole grain mustard, Dijon mustard, olive oil, apple cider vinegar, minced garlic, smoked paprika, salt, pepper, and thyme. The mixture should look glossy and smell pungent yet sweet, with the mustard seeds visible throughout.

Prepare the chicken: Place the chicken breasts in a single layer in your 9x13 inch baking dish. If any pieces are much thicker than the others, gently pound them to an even thickness using a rolling pin or the bottom of a sturdy mug. You'll hear a soft thud as you flatten them.

Coat the chicken: Pour the honey mustard mixture over the chicken, turning each piece to ensure it's fully coated. The chicken should look glossy and well-covered, with bits of mustard seed clinging to the surface.

Bake uncovered: Place the dish in the preheated oven and bake for 25 minutes. At this point, the kitchen will smell sweet and tangy, and the chicken will look pale but starting to firm up around the edges.

Baste and finish: Remove the dish from the oven and spoon the pan juices over the chicken. Return to the oven for another 10-15 minutes. The chicken is done when it reaches an internal temperature of 74°C (165°F) and the thickest part feels firm to the touch but yields slightly when pressed. The glaze should be bubbling and sticky, with deep golden-brown edges.

Rest and serve: Remove the dish from the oven and let the chicken rest for 5 minutes. You'll notice the juices settling back into the meat rather than pooling on the plate. Garnish with fresh parsley if desired, then serve with the pan juices spooned over the top.