Make the pastry. Combine the flour, sugar, salt and butter in a food processor until combined and looks like sand. Then add the cold water, just enough to let the ingredients combine. Place in plastic wrap, flatten to a disc, wrap securely and refrigerate for at least 30 minutes.
Preheat the oven to 350 degrees (180 Celsius).
Prepare the pear(s). Peel, core and slice length-wise, thinly.
Over medium heat in a skillet, add 1-2 tablespoons of butter and add the sliced pears. Cook until just barely soft about 5 minutes. While the pears are cooking add three sprigs of thyme, muscavado sugar and the liqueur. Once the pears are tender, turn off the heat, remove the pears, discard the thyme and keep the sauce in the pan. You will be cooking and reducing down soon. Set aside.
Roll out the pastry and place on a parchment lined baking sheet. Position the pears as you desire, in a circular design or if cubed in a pile in the middle leaving at least two inches around the edge of the pastry free of pear.
Pull the leaves of the remaining 3 sprigs of thyme off the stems (about 1 tbsp) and sprinkle over the top of the pears. Fold the edges of the pastry around the edges of the pears. Set aside for a minute while you reduce the pear liquid.
Over medium heat, in the same pan you cooked the pears, the juice should still remain. Cook it until it becomes the consistency of syrup. Then pour over the pears already situated in your tart (before you put it in the oven to cook).
Place the galette in the oven and bake for 30 minutes (40 minutes if large).
Bake until the pastry is golden brown. Remove and enjoy.