Set an oven rack in the upper-middle position and heat the oven to 425°F. Line a large rimmed sheet pan with parchment paper or foil for easier cleanup.

Put the chicken thighs on a plate and pat them dry well with paper towels. Dry skin browns better in the oven.

In a large bowl, whisk together the olive oil, Dijon mustard, kosher salt, and black pepper until the mixture looks smooth.

Put the broccoli in a medium bowl. Spoon 1 tablespoon of the mustard mixture over it and toss to coat, then set the bowl aside.

Put the chicken thighs into the large bowl with the remaining mustard mixture. Turn the chicken until it is coated on all sides.

Add the red onion wedges and drained butter beans to the same large bowl. Toss gently so the onions and beans pick up the mustard mixture left in the bowl.

Spread the onion wedges and butter beans across the lined sheet pan in a single layer. Leave some open space for the broccoli you will add later.

Set the chicken thighs skin-side up on the sheet pan, spacing them over the onions and beans. Give each piece a little room so the skin can roast instead of steam.

Tuck the thin lemon slices around the chicken and vegetables on the sheet pan. Keep most of the slices exposed so they can roast and lightly brown.

Put the sheet pan in the oven and roast for 15 minutes. The chicken will not be fully cooked yet, but the skin should start to look glossy and lightly golden.

Take the sheet pan out of the oven and set it on the stove. Use a spoon to stir the beans and onions so the bottom side does not get too dark.

Add the broccoli to the open spaces on the sheet pan. Spread the florets out so as many cut sides as possible touch the pan.