Thaw 1 sheet of Rolled Puff Pastry according to package directions. The other can be frozen for future use.
Preheat oven to 350°F.
Place the butter, brown sugar and salt in a small saucepan over medium high heat. Cook for 5 minutes, stirring occasionally. Remove from heat and stir in 1 Tbsp. grainy mustard and thyme.
Cut the rolled puff pastry sheet into 4 equal squares and transfer to baking tray. Score a 1.5cm border on each piece. Brush the borders of the pastry with the egg wash. Brush the centres with the remaining 2 Tbsp. grainy mustard.
For fanned out pears, cut pears into quarters and remove the core. Cut into thin slices. For hassleback pear halves, cut pears in half and remove the core. Cut thin slices into the pear but leaving it attached at the top. Place skin side up on the pastry.
Sprinkle 1 Tbsp. cheese over each pastry square and place pear or pear slices in the center.
Bake for 20 to 25 minutes, until deep golden brown. Sprinkle the tarts with walnuts and the remaining cheese and drizzle with the sauce.