pasta with yogurt and carmelized onions

http://food52.com/recipes/24867-diane-kochilas-pasta-with-yogurt-and-caramelized-onions?utm_source=FOOD52+Subscribers+List&utm_campaign=a0e89ceaa6-Midweek_Edit_Only_1_8_2014&utm_medium=email&utm_term=0_191568346e-a0e89ceaa6-18705781

• 5 tablespoons extra-virgin olive oil
• 6 cups coarsely chopped onions
• Sea salt
• 1 pound tagliatelle or other fresh pasta
• 2 cups thick, strained Greek-style yogurt (see note)
• 1 cup coarsely grated kefalotyri cheese, or pecorino Romano

1. Heat the olive oil in a large skillet over medium-high heat and add the onions. Reduce the heat to medium low and cook, stirring frequently and seasoning with salt to taste as you go, until the onions are soft and golden brown, 20 to 30 minutes.
2. Meanwhile, fill a large pot with water and bring to a boil. As the water heats, add enough salt so that you can taste it. Add the pasta and cook until soft, not al dente. Reserve 1/2 cup pasta water. Combine the yogurt with 1/4 cup cooking water and mix well. Add more of the reserved pasta water as needed to get the sauce to your thickness. Drain the pasta and toss with the yogurt mixture.
3. Serve the pasta immediately, sprinkled generously with cheese and topped with caramelized onions and their juices.
4. Note: If not using thick, Greek yogurt, line a colander with cheesecloth and set over a bowl or in the sink. Add the yogurt and let drain for 2 hours before proceeding with step 1. For a treat, seek out sheep's milk yogurt for this.