Preheat your oven to 400°F (200°C). Line an extra-large rimmed half-sheet baking pan with parchment paper or heavy-duty foil for effortless cleanup.

In a medium mixing bowl, whisk together the honey, whole grain Dijon mustard, smooth mustard, minced garlic, apple cider vinegar, chopped rosemary, and a pinch of salt until smooth and glossy. Reserve 3 tablespoons of the glaze in a clean small bowl for a final finishing brush after baking.

Toss the cut potatoes and red onions with 1 tablespoon olive oil, 1/2 teaspoon kosher salt, garlic powder, and black pepper. Spread them out across the prepared sheet pan, leaving spaces for the chicken cutlets. (Potatoes roast fastest when their cut sides are placed flat against the pan).

Pat chicken thighs dry. Arrange them onto the sheet pan between the potatoes. Brush the remaining honey mustard glaze generously over both sides of each chicken thigh. Roast in the preheated oven for 15 minutes.

Carefully pull out the sheet pan, toss in the broccoli florets tossed in the remaining 1 tablespoon olive oil, and brush chicken with another layer of glaze. Return to the oven for 10 to 12 minutes until chicken reaches 175°F (80°C) internally and the edges caramelize into sticky golden perfection. Brush with reserved glaze, garnish with fresh parsley, and serve!