Cook rotini according to package directions. Add the asparagus and broccoli during the last 2 minutes of cooking. Reserve 1/4 cup (60 mL) pasta water; drain well.
Place onion, spinach and oil in a blender; purée until smooth. Transfer the mixture to a large skillet set over medium heat. Add cooked rotini, pasta water, asparagus, broccoli, peas, lemon juice, salt and pepper. Cook, stirring often, for 3 minutes or until heated through. Adjust salt and pepper to taste.