HEAT OVEN TO 250F/121C. Ideally, it's best to bring the pork roast to room temperature 30-45 min ahead for more even cooking
PREPARE PORK ROAST: Mix all seasoning ingredients in a small bowl. Smear 2/3 of it all over the pork. Save the rest to toss with the veggies. Place roast, fat side up, in a large cast iron skillet or roasting pan, sprayed with oil.
ADD VEGGIES (optional): Place cut up onions, potatoes, garlic and carrots around the pork in the skillet/pan. Add the remaining seasoning mixture to the veggies and mix to combine (I use my clean hands). Add 1/2 cup broth to pan.
START ROASTING AT LOW TEMP: Roast pork and veggies (if using) in oven for about 1 - 1 1/4 hours or until temperature on probe or instant thermometer reaches 135F/57.2 for medium (you'll be cooking it longer later on). Or 140F/60C for slightly more well done. Transfer pork to a plate, cover loosely with aluminum foil and let it rest for at least 20 minutes and up to 45 minutes.
MAKE ROAST PORK GRAVY RECIPE: While pork is roasting, make gravy. Put oil (or fat drippings) into a small saucepan. Heat to medium heat. Stir in flour and cook, stirring, for 2 minutes until it starts to turn a very light brown. (It should be a loose paste. If too dry, add a bit more oil). Add broth and thyme and whisk/stir until smooth - about 3 minutes. Gravy will thicken. Taste and adjust seasonings (salt, pepper, more thyme). For a browner color, a tsp of Worcestershire or soy sauce. Taste and add salt and pepper as needed.
FINISH ROASTING AT HIGH TEMP: While roast is resting, increase oven temperature to 500F/260C. °If cooking roast with veggies, stir veggies in pan and place back into the oven for 15-20 minutes to brown (no need to wait until high temperature is reached). °When pork roast has rested, place roast back in pan with veggies and roast at high heat for 7-10 minutes until exterior browns nicely. Internal temperature of meat should rise to 145F/62.8 for medium (just slightly pink). Or150F/65.6C for a bit more done. Over cooking will make the pork dry.
SLICE AND SERVE: Slice boneless pork roast (about 1/4 inch thick) on cutting board. Serve along with veggies, if using, and gravy. Enjoy!