Prepare pasta according to package directions to al dente. Once done, reserve 1 cup pasta water for thinning sauce later, if needed.
Meanwhile, in a large nonstick skillet sprayed with cooking spray, heat one tablespoon oil over medium-high heat Add onion and saute until softened and browned, about 4 minutes. Add garlic and saute for one minute, stirring constantly. Stir in spinach and cook until heated throughout, about 2 minutes.
Pour spinach mixture into a food processor. Add one tablespoon oil, ¼ cup Parmesan cheese, yogurt and ground mustard. Process until smooth.
Put cooked pasta in a large bowl. Pour sauce over pasta and toss to coat. Sprinkle with crushed red pepper flakes and remaining Parmesan cheese and toss again. Serve warm.
Recipe by Alida's Kitchen at https://www.alidaskitchen.com/2015/01/23/spinach-pesto-pasta-recipe/