Preheat the oven to 400°F (200°C).
Wash and dry 2 pounds sweet potatoes. Slice thin slits using a large kitchen knife, about ⅛ inch (3 mm) apart, stopping just before cutting all the way through. You can place two wooden spoons or chopsticks on the sides of each potato to anchor it and stop the knife from slicing all the way through.
Place sweet potatoes, cut side up, on a foil-lined baking sheet (easier clean-up, but you can also just butter or oil the pan). Brush the potatoes with some of the 4 tablespoons olive oil or melted unsalted butter, making sure some drips into the slits. Season with ½ teaspoon sea or kosher salt, ⅛ teaspoons freshly ground black pepper, and ½ teaspoon cinnamon (if using). The honey butter also has cinnamon, so you can skip it here if you don't want a lighter-spiced flavor.
Roast for 45–55 minutes, brushing with more oil/butter halfway through, until the potatoes are tender inside and the edges are crispy.
Make the sweetened butter:
In a bowl, mix ½ cup unsalted butter, melted, with 2-3 tablespoons runny honey, ½ teaspoon ground cinnamon, and a pinch of salt until smooth.
Stir in ½ cup walnuts or pecans, chopped and lightly toasted. Taste and adjust sweetness or salt to preference. Once potatoes pierce easily and are cooked, remove from the oven.
Spread or brush the honey butter over the hot potatoes. You can serve the whole pieces hasselback-style or slice them all the way through and serve rounds.
Garnish with extra walnuts and chopped fresh thyme or sage, or other herb of choice if desired. Serve warm.