Cut butter into 8 pieces. Place in a medium saucepan over medium heat (ideally you want to use a light colored pan such as stainless steel or enameled cast iron so you can see the bottom). Whisk the butter as it melts and begins to foam. Continue whisking until brown bits begin to form on the bottom of the pan and the butter has lightly browned.

Remove from the heat and stir in the sage leaves and garlic. Transfer the mixture to a bowl, making sure to scrape up and include any brown bits from the pan. Set aside to let the flavors infuse with the brown butter.

Slice off a small amount lengthwise from the bottom of each potato to create a stable surface that won't roll. Cut the sweet potato into approximately 1/4-inch slices, leaving 1/8 inch intact at the bottom. (Note: to make this easier, place chopsticks on either side of the potato to prevent the knife from cutting all the way through).

Remove the sage leaves from the brown butter and discard. Brush some of the butter on the bottom of a 9x13-inch baking dish and top with the sweet potatoes.

Drizzle the remaining brown butter over the potatoes, using a pastry brush to cover the tops and in between some of the potato slices.

Combine the chopped sage, salt, pepper and nutmeg in a small bowl and sprinkle evenly over the potatoes, pressing some in between the potato slices.

Roast until the sweet potatoes are soft in the middle and easily pierced with a toothpick, approximately 45 to 60 minutes.