Heat oven to 400°F. Combine 1/2 teaspoon salt, 1/4 teaspoon pepper flakes, 1/4 teaspoon black pepper and 1 teaspoon oil in cup. Rub on pork.
Heat cast-iron skillet over medium-high heat. Add pork; sear on all sides, 5 minutes. Transfer pan to 400°F oven. Roast 15 to 20 minutes, until internal temperature is 145°F.
Meanwhile, cook spaghetti in a large pot of boiling water for 7 minutes; drain; transfer to bowl.
For sauce, whisk remaining 1/4 teaspoon salt, 1/4 teaspoon pepper flakes, 1/4 teaspoon pepper, 2 teaspoons oil, rind, scallions, juice, soy sauce, cornstarch, vinegar, garlic and ginger in small bowl.
Remove cooked pork from oven; let stand 10 minutes.
Add sauce to skillet; cook over medium heat 3 to 5 minutes, until bubbly and thickened. Add orange sections and watercress to spaghetti in bowl. Thinly slice pork; halve any large pieces. Add to bowl along with sauce. Toss to coat and serve.