Step 1

Bring a large pot of salted water to a boil.

Step 2

Heat 4 tablespoons of the butter in a medium Dutch oven over medium-high heat. Add the chicken and sprinkle with a big pinch of salt and pepper. Cook until browned and cooked through, 5 minutes. Remove with a slotted spoon. Lower the heat to medium and add the remaining 1 tablespoon butter along with the garlic and onion. Cook, stirring occasionally, until softened, 2 to 3 minutes. Sprinkle in the flour and stir to coat the vegetables. Cook, stirring, until the coating is smooth but not browned, about 2 minutes.

Step 3

Add the chicken broth, heavy cream, 1 cup of water and 1/2 teaspoon salt to the pot and stir to combine. Adjust the heat so the soup is simmering and cook until slightly thickened, about 10 minutes.

Step 4

Meanwhile, add the fettucine to the boiling water, breaking it into 2-inch pieces as you go. Cook according to the package instructions until just al dente, adding the broccoli to the pot in the last 3 minutes of cooking. Drain well, then stir the pasta and broccoli into the pot along with the chicken. Stir in the parsley. Remove from the heat and stir in the Parmesan and lemon juice.

Step 5

Serve in bowls then garnish with more Parmesan, parsley, lemon slices and red pepper flakes, if desired.