Preheat oven to 375 °.
Generously butter a baking dish and set it aside.
Cover the sheet pans with foil. Prick the sweet potato with a fork on all sides, about 8-10 times. The sweet potatoes will leach some liquid as they bake. The foil will prevent them from sticking to the pan. Put in the oven for 50 minutes.
Core three Granny Smith apples and cut in half. Lay cut side down on the sheet pans. Place in the oven when the sweet potatoes have baked for 30 minutes.
After 50 minutes take both sheet pans out of the oven. The apples will appear to have almost melted. Remove the peel and remove any cooked flesh that still adheres to it. Place the apple flesh and all that is on the sheet pan into a medium-sized bowl. If there is any juice from the apples, pour that into the bowl as well. The only thing you are not using is the peel.
When the sweet potatoes have cooled enough to handle them, remove and discard the peel. Put the sweet potato in the same bowl as the apples. Mash together. How chunky is up to you.
Combine ¼ cup of melted butter, ¼ cup of maple syrup, 1 ½ teaspoon of kosher salt, ½ teaspoon of ground black pepper, ½ teaspoon of ground nutmeg, and ¼ teaspoon of lemon juice.
Add the butter mixture to the mashed sweet potatoes/apples. Once completely mixed, spoon into the prepared baking dish.
Crumble the streusel on top of the sweet potato/apple mash. Cover the entire surface. You will probably have streusel leftover. It can be frozen and used at a later time.
Cover the baking dish with foil. Place in the oven and bake for 40 minutes.
After 40 minutes, remove the foil and put the baking dish back in the oven for approximately 20 minutes. Make sure the streusel has browned and that the interior of the casserole is 165 degrees. If it is not, put the baking dish back in the oven. Check after 5 minutes to see if the top is brown.
The casserole can sit for 10-15 minutes before serving. It retains its heat for a while.