Cook pasta in 2 quarts boiling, salted water until al dente, 7-8 minutes. Transfer pasta to large bowl and toss with dressing. Keep warm.
Heat olive oil in skillet over medium-high heat. Sauté onions 3-4 minutes until they begin to become translucent. Add garlic and sauté 1 minute longer, stirring constantly.
Add spinach to skillet and sauté until just beginning to wilt, 2-3 minutes. Combine with pasta and mix in ricotta cheese. Season with salt and pepper. Top with toasted pecans.