To Prep the Potatoes: Preheat oven to 450F. Line a large, rimmed baking pan with parchment paper.
Bring a large pot of salted water to a boil. Add the peeled and chopped potatoes to cook until fork tender, about 8-10 minutes. Strain and let steam dry for 5-10 minutes.
To Prep the Rub: In the meantime, make the citrus fennel rub by combining the sugar, orange zest and chopped sage in a small bowl. Rub with your fingers until the sugar turns an orange-y color. This helps to release the essential oils of the sage and orange.
Add the crushed dried fennel, olive oil, and Dijon. Mix until well combined. Set aside.
Prepare the potatoes for their first roast: Transfer the potatoes to a large bowl. Add the chopped sage, then season generously with black pepper and salt. Move around the bowl to toss the potatoes vigorously in the bowl to roughen up the edges, about 30-40 seconds.
Add the potatoes to the prepared baking pan, spread out evenly. Drizzle generously with the 4 tablespoons of olive oil. Bake for 15 minutes.
In the meantime, prep the chicken to bake. If needed, pound the chicken breasts with a kitchen mallet to 1” thick. Slice a large slit lengthwise so that it opens like a book. Add 1/3rd of the soft spreadable cheese on one side of the slit, then cover with the flap to seal. It’s okay if some of the cheese is showing!
If desired, layer fennel and orange slices in a rimmed baking dish for the chicken.
Add the stuffed chicken to a rimmed baking dish. Pat the surface of the chicken breast dry with a paper towel, then season with salt and black pepper.
Pat the citrus fennel rub evenly all over the surface of each chicken breast.
Baking the chicken and potatoes together: Reduce oven heat to 425F. Take out the potatoes to toss with a spatula. Then place back in the oven on a lower rack.
Add the chicken baking dish to the upper/middle rack to roast for 20-25 minutes, until juices are flowing clear and fully cooked through.
While they bake, make the salad. Toss all ingredients for the shaved brussels sprout salad together. Season with salt and black pepper to taste.
Let the chicken rest at least 5-8 minutes before serving warm with the crispy potatoes and shaved brussels sprouts salad.