Cook pasta according to package directions; reserve 1/2 cup (125 mL) pasta water.
Meanwhile, in large pot of boiling salted water, cook sweet potato for about 2 minutes or until tender-crisp; drain well.
Heat oil in large nonstick skillet set over medium heat; cook sweet potato, garlic, salt, pepper and hot pepper flakes for about 3 minutes or until sweet potato is tender. Stir in spinach and lemon juice; cook for 1 to 2 minutes or until spinach starts to wilt.
Toss pasta with sweet potato mixture, reserved pasta water, ricotta cheese and lemon zest. Sprinkle with pine nuts.
Substitute butternut squash for sweet potato if desired.