1. Preheat oven to 450°. Sprinkle 1 tsp. salt and 1/2 tsp. pepper over pork; rub rosemary and 1 Tbsp. thyme over pork. Place pork in a lightly greased roasting pan.
2. Toss together onions, olive oil, 1/8 tsp. pepper, and remaining 1/4 tsp. salt until coated. Arrange onions around pork.
3. Bake at 450° for 30 minutes; reduce oven temperature to 375°. Bake 50 more minutes or until a meat thermometer inserted into thickest portion registers 145°, stirring onions once. Transfer pork to a cutting board; cover loosely with aluminum foil, and let stand 10 minutes before slicing.
4. Meanwhile, coarsely chop onions; transfer to a bowl. Stir in vinegar, brown sugar, and remaining 1 tsp. thyme. Stir in pumpkin seeds before serving. Serve pork with relish.