Cook pasta in 2 quarts boiling, salted water until al dente, 7 to 8 minutes. Transfer pasta to a large bowl and toss with dressing. Keep warm. Heat olive oil in a skillet over medium-high heat. Sauté onions 3 to 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 to 3 minutes. Combine with pasta and mix in ricotta cheese. Season with salt and pepper. Top with toasted pecans.