Bring a large pot of salted water to a boil. Add the pasta and cook until al dente. Reserve 1 1/4 cups pasta water then drain the pasta and set it aside.
Add 1 tablespoon vegetable oil to a heavy-bottomed stock pot set over medium-high. Once the oil is hot, add the chicken, season it with salt and pepper then brown it on all sides until it is cooked through. Using a slotted spoon, remove the chicken to a bowl and set it aside.
Reduce the heat to medium-low. Add the remaining 2 tablespoons vegetable oil to the skillet then add the onions and garlic and cook, stirring as needed, until the onions are caramelized, about 30 minutes.
Increase the heat to high then add the sherry, and cook while scraping up any brown bits, for 1 minute. Stir in the flour then cook for 2 minutes.
Stir in the beef bouillon paste, Worcestershire sauce, heavy cream and 1 ¼ cups pasta water. Simmer the sauce for 2 minutes.
Stir in the pasta, browned chicken and thyme. Bring the mixture to a simmer and cook until the sauce has thickened slightly, about 5 minutes, then serve.
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