Special equipment: an 8-by-8-inch baking pan
Preheat the oven to 375 degrees F.
Place a layer of sweet potatoes in an 8-by-8-inch baking pan. Sprinkle with half of the agave, honey, brown sugar, nutmeg and lemon zest and a pinch kosher salt.
Add another layer of potatoes. Sprinkle with the remaining agave, honey, brown sugar, nutmeg and lemon zest and another pinch of salt.
Dot the potatoes with the butter. Bake until fork-tender, 25 minutes.