Preheat the oven to 450 degrees . Season the pork with salt and pepper. In an ovenproof skillet, heat 2 tbsp. EVOO over medium. Add the pork and cook until browned on all sides, about 8 minutes. Cover the pork loosely with foil, then transfer the skillet to the oven. Roast until the pork registers 145 degrees on an instant-read thermometer, 10 to 13 minutes. Let rest while you make the dressing.
Wipe out the skillet, then heat the remaining 2 tbsp. EVOO over medium. Add the shallot and cook until softened, about 2 minutes. Stir in the vinegar, mustard, honey and a pinch of salt, scraping up any browned bits from the bottom of the pan. Remove from the heat.
Cut the pork into 1/2-inch-thick slices, then into strips.
In a medium bowl, toss the pork, spinach, pear and red onion with the warm dressing; season.