Preheat the oven to 475°F. Slice sweet potatoes into 1-inch-thick rounds, keeping them uniform for even cooking.
In a skillet, melt 4 tablespoons of butter over medium heat until it foams and turns golden brown, releasing a nutty aroma. Remove from heat immediately and stir in the maple syrup.
Whisk the maple brown butter with avocado oil, salt, and pepper.
In a metal baking dish or on a rimmed sheet pan, toss sweet potato slices with the glaze until well coated. Arrange in a single layer.
Roast for 15 minutes, then flip each slice carefully. Roast another 15 minutes until caramelized.
Scatter garlic (and thyme if using) over the potatoes. Warm the broth, then pour evenly over top. Dot with the remaining tablespoon of butter.
Return to the oven for 10–15 minutes, until most of the broth reduces to a glaze and the potatoes are fork-tender.
Transfer to a serving dish, drizzle pan glaze over top, and sprinkle with nuts if desired. Serve warm.