Preheat the oven to 400 degrees F. Combine the peaches, brown sugar, ketchup, vinegar, soy sauce, garlic and ginger in a food processor or blender.
Pat the pork dry with paper towels and sprinkle liberally with salt and pepper. Reserve half (about 2 cups) of the peach BBQ sauce and set aside. Transfer the pork, fat-side up, to a rack over a heavy-bottomed roasting pan and brush the rest of the sauce all over the pork. Add the stock to the bottom of the roasting pan.
Roast the pork until a meat thermometer inserted into the thickest part reads 140 degrees F, about 1 hour. Transfer the pork to a baking sheet to rest for 10 minutes. Reserve 1 1/4 pounds for the stir fry, and reserve the rest for leftovers or another recipe.
Remove the rack from the roasting pan and place on the stove over medium-high heat. Whisk in the reserved BBQ sauce and scrape the bottom of the pan to incorporate the browned sauce. Cook until slightly reduced and thickened, about 10 minutes.
For the stir fry:
Cook the rice according to the package directions.
Add 2 tablespoons of the sesame oil to a small saute pan and heat over medium heat until it begins to shimmer; turn off the heat, add the five-spice powder and cool to room temperature.
Meanwhile, whisk together the soy sauce, vinegar, orange juice, sugar and cornstarch in a small bowl.
Slice the pork into 1/4-inch rounds, then slice again to make 1/2-inch-wide strips. Place the pork in a medium bowl and season with the sesame oil and five-spice mixture.
Heat the remaining 1 tablespoon oil over high heat until very hot in a large skillet or wok. Add the peas, onions and bell peppers and stir-fry until the vegetables begin to soften, about 1 minute. Add the ginger and garlic and continue to stir-fry for an additional minute. Finally, add the sliced pork, the soy-vinegar mixture and scallions and continue to cook until the pork has warmed through, 1 to 2 minutes.
Cook until the sauce thickens, 1 to 2 minutes. Turn off the heat and give it a few more stirs. Serve over the rice and garnish with roasted peanuts and mandarin oranges, if desired.