Black Pepper and Onion Spaghetti

Preparation

1. Step 1

   In a large Dutch oven or skillet with a lid, add the olive oil and white and red onions and set over medium heat. Once sizzling, cook until onions are slightly wilted and starting to sweat but not browned, 5 to 7 minutes. Stir in ½ teaspoon salt, cover, reduce heat to low and simmer, stirring occasionally, until the onions are very soft and translucent but mostly still in strands, about 1 hour. Adjust heat as needed to maintain a simmer, and if the onions are burning, add a few tablespoons of water. (Onions can be made ahead and refrigerated for 1 week or frozen for a couple months.)

2. Step 2

   When you’re ready to serve, bring a large pot of salted water to a boil. Add the spaghetti and cook until al dente according to package directions. Reserve 1½ cups pasta water and drain.

3. Step 3

   Add ½ cup pasta water, the pasta and the tarragon leaves to the onions. Raise heat to medium-high and stir vigorously until the pasta is well coated in sauce, 1 to 2 minutes. Add more pasta water as needed until the noodles are glossy. Season to taste with salt.

4. Step 4

   Transfer the pasta to plates or shallow bowls and cover with cheese (if using) and generous grinds of black pepper.