Preheat oven to 375 degrees F.
Put sweet potatoes in a glass or stainless steel bowl.
Melt the butter in a small saucepan over low heat. Stir in brown sugar and lemon juice. When sugar is completely melted, stir in Toasted Spice Rub and gray salt. Pour mixture over sweet potatoes. Toss well.
Arrange sweet potatoes in a baking dish, cut side up. With rubber spatula, scrape all the butter and spice mixture over the top of the sweet potatoes, and then add a sprinkling of Toasted Spice Rub on top of each one. Cover with foil and bake at 375 degrees F until easily pierced with a knife.
Toast the fennel seeds, coriander seeds, and peppercorns in a small, heavy pan over medium heat. When the fennel turns light brown, work quickly. Turn on the exhaust fan, add the red pepper flakes, and toss, toss, toss, always under the fan. Immediately turn the spice mixture out onto a plate to cool. Put in a blender with the chili powder, salt, and cinnamon and blend until the spices are evenly ground. If you have a small spice mill or a coffee grinder dedicated to grinding spices, grind only the fennel, coriander, pepper, and chili flakes. Pour into a bowl and toss with the remaining ingredients.