Directions

1. In a large skillet, heat 1 tablespoon oil over medium; season pork with salt and pepper. Add pork to skillet, and cook, turning occasionally, until an instant-read thermometer inserted in thickest part registers 145 degrees, 20 to 25 minutes (reduce heat if pork browns too quickly). Transfer pork to a plate, and cover loosely with aluminum foil; let rest 5 to 10 minutes (reserve skillet).
2. Meanwhile, in a blender, combine orange juice, mustard, honey, garlic, and 2 tablespoons oil. Season with salt and pepper, and blend until smooth; set vinaigrette aside.
3. In reserved skillet (if bottom of skillet is blackened, use a new one), heat remaining tablespoon oil over high. Add as much spinach as will fit; toss until wilted, adding more spinach as there is room, 3 to 5 minutes total. Drain off excess liquid, and season spinach with salt and pepper. Slice pork and drizzle with vinaigrette; serve with spinach.