Step 1: Set up your grill for indirect grilling and build a medium-hot fire (400°F). Place the salt slab in the center of the grate away from the heat and preheat it as well.
Step 2: Cut the squashes in half widthwise (through the stem end) and scoop out the seeds. Place each squash half, cut sides down, on the salt slab. Roast until tender (squeeze the sides to check for doneness), about 45 minutes to 1 hour, depending on the size of the squash.
Step 3: Meanwhile, make the stuffing: Place the bacon and shallots in a cool cast iron skillet. Cook over medium heat until the bacon begins to brown, 3 to 5 minutes, stirring often. Add the pecans and cranberries and cook until the nuts are toasted and fragrant, 2 minutes. Remove the pan from the heat and let the mixture cool, then stir in the cheese.
Step 4: Turn the squash halves cut sides up (grill rings will help them hold them steady). Spoon the stuffing into the squash halves. Pour 1 tablespoon or more of the maple syrup over each squash half and place 1/2 tablespoon of butter in the center. Return the squash halves to the grill.
Step 5: Continue salt grilling the squash until the butter melts and the filling sizzles, 5 to 8 minutes.
Note: For a vegetarian version, omit the bacon and sauté the shallots, nuts, and cranberries in 2 tablespoons of butter.