Cover the sweet potatoes with water and simmer about 12 minutes, then drain. Mix with half and half, maple syrup, vanilla, salt and egg. Spread into a 9x13" pan.
Use a food processor or mixer to make the streusel. Combine all the ingredients except the pecans and pulse, then mix in the pecans. Sprinkle across the top of the sweet potatoes.
You can do all this ahead of time and freeze if you want. Or do the night before and refrigerate. If you've frozen it, thaw before baking.
Bake at 350 for half an hour (a bit longer if cold).