Preheat your oven to 425 degrees F.

While waiting for your oven to preheat, add your butter to a small bowl and microwave in 20 second increments until melted.

Zest the grapefruit over the bowl of melted butter. Whisk in the white miso and mirin.

Add your chicken to a 9×13 baking pan with at least 1-inch sides. Season with salt and pepper.

Scrape the miso butter on top of the chicken and toss to combine evenly in the butter.

Arrange the chicken in a single layer and add to your preheated oven for 25 minutes.

While waiting for the chicken to cook, cut off the rind of the grapefruit using a paring knife. Cut the grapefruit flesh into bite sized pieces and reserve.

After 25 minutes, turn on your broiler and broil for 1 to 2 minutes until browned on top to your liking. (Be sure to watch the chicken while broiling, it can go from nicely browned to burnt in seconds!)

Add the arugula to a serving dish and place the cooked chicken on top.

Add the grapefruit pieces to the baking pan along with the juices from the chicken and stir to combine.

Top the chicken and arugula with sauce from the pan and serve.