Preheat the oven to 425 degrees . Season the pork all over with salt and pepper. Place in the center of a roasting pan.
In a small bowl, stir together the mustard, vinegar and brown sugar and brush over the pork. Scatter the brussels sprouts, sweet potatoes and thyme around the pork. Sprinkle with the oil, any remaining mustard glaze, salt and pepper and stir to coat.
Transfer to the oven and roast, tossing the vegetables occasionally, until almost tender, about 30 minutes. (If they stick to the pan, add 2 tbsp. water; stir.)
Stir in the pear and roast until the vegetables are tender and an instant-read thermometer inserted into the thickest part of the pork registers 150 degrees ;15 to 20 minutes. Tent with foil if the roast starts to get too dark. Discard the thyme. Let the roast rest for 5 minutes, then thinly slice and serve with the vegetables. Garnish with more thyme and serve with more mustard.