Preheat the oven to 350℉.
In a small bowl mix together the brown sugar, salt, cinnamon, and orange zest. Slice the vanilla pod in half and scrape out the seeds. Add them to the bowl and reserve the pod.
Squeeze the juice of the orange into the bowl. Add the Grand Marnier. Mix until combined.
Spread the pieces of butter over the bottom of a 9x13-inch baking pan. Add the reserved vanilla pod.
Pour the liquid into the pan.
Core the pears and scoop out some of the flesh to create a small hollow. Rub the pear halves with lemon.
Add them to the pan with the sliced side facing up. Scatter the thyme sprigs over the pears.
Roast the pears for 20 to 30 minutes. Baste them with the cooking liquid every 5 minutes. I like my pears to still be pretty firm.
Let them cool for at least 1 hour. At this point, you can also cover and chill them until the following day.