Bring a large pot of water to a boil, salt the water and cook the pasta al dente.
While the pasta is cooking, in a fry pan over medium-low heat, warm the butter. Add the onion, garlic and sautué until very soft and lightly golden, about 15 minutes. Add the shredded chicken and tomato paste. Stir and cook for 5 minutes more.
In a saucepan over medium heat, bring the milk to just below a boil. In another saucepan over medium-high heat, melt the butter. Stir the flour into the butter and cook, stirring, for 2 minutes.
Remove from the heat and gradually whisk in the hot milk. Set over medium-low heat and cook, stirring, until thick and smooth, 3 to 4 minutes. Season with salt, pepper and nutmeg. Remove the sauce from the heat. Add the cheese
Drain pasta and add to the skillet with the shredded chicken. Toss the pasta to coat evenly with the sauce. Top with cheese and cilantro. Serve immediately.