Preheat the oven to 450°F. Grease a 12x9 inch baking sheet with cooking spray. Set it aside.

Wash and peel the sweet potatoes and cut them into 1-inch cubes.

Wash the brussels sprouts, cut off the ends and cut them in half lengthwise.

Place the sweet potatoes on the greased baking sheet and drizzle them with olive oil and ¼ teaspoon of sea salt. Roast in the oven for 15 minutes.

Place the butter, thyme and sage leaves in a large deep sauté pan. Melt the butter over medium-low heat. Keep whisking until the butter stops foaming and turns deep brown. This takes about 5-7 minutes. You'll see lots of brown specks at the bottom of your sauté pan. 

Add the brussels sprouts to the sauté pan, raise the heat slightly and and toss them in brown butter. Sauté until the cut sides start to brown then turn the flame off.

Remove the sweet potatoes from the oven and transfer them to a 9x13-inch casserole pan or glass baking dish. Make sure to drizzle any olive oil that's left on the baking sheet over the sweet potatoes.

Transfer the brussels sprouts, brown butter, thyme and sage from the skillet to the baking dish with the sweet potatoes. Sprinkle them with black pepper, another pinch of sea salt and gently toss until everything is coated.

Sprinkle the brown sugar over the brussels sprouts and sweet potatoes. Bake in the oven for 15 minutes and then place them under the broiler for 3-5 minutes - until the edges of the brussels sprouts become crispy.