Preheat oven to 400°F. Stir together harissa, 1 tbsp. olive oil, lemon juice and half of the salt; rub all over pork tenderloin.
Transfer to foil-lined baking sheet and let stand for 15 minutes.
Roast for 15 to 20 minutes or until internal temperature reaches 160°F. Tent with foil and let stand for 10 minutes. Slice crosswise.
Meanwhile, cook pasta according to package directions; drain well.
Heat remaining oil in skillet set over medium-high heat; cook onion for 5 minutes. Add spinach, orange zest, remaining salt and chicken broth; cook for about 3 minutes until spinach wilts. Add tagliatelli and toss to coat. Serve with pork tenderloin slices.
Look for harissa paste in the condiment aisle, where specialty Middle Eastern foods are sold or online.