Preheat the oven to 375 degrees F.
Place the sweet potatoes on a foil lined baking sheet and roast for 45 minutes to 1 hour, until very soft to the touch. Remove from the oven and let cool 20 minutes. When cool enough to handle, peel them by hand and put the flesh in the bowl of a mixer. Using a paddle attachment, mix in the lemon zest and juice, maple syrup and brown sugar.
Place the bourbon in a small saucepan and place over high heat. Let it come just to the boil and then tilt the pan slightly towards you to set it aflame*. Add to the potatoes along with the butter. Mix well. Add salt and pepper and transfer to a 13 by 9-inch oven-safe casserole dish. (Recipe can be made to this point up to 2 days before, refrigerated.) Sprinkle topping over potatoes and bake for 20 minutes until the top is golden brown.
Alternatively, you may simply sprinkle the top of the casserole with a little bit of brown sugar and 1/2 cup chopped pecans.
Mix the flour, brown sugar, salt, pepper, thyme and pecans together in a small bowl. Add the butter and work with your fingers until a crumbly mass forms.
*This method does not "burn off" the alcohol.