Begin by cutting the chicken breast into 1-inch bite-sized pieces and seasoning them generously with kosher salt and freshly cracked black pepper. Set aside.

Fill a large pot with salted water and bring it to a boil over high heat.

In a large skillet over medium-high heat, melt 1 tablespoon of unsalted butter. Add the seasoned chicken pieces and cook for about 3 minutes until golden brown. Flip and cook for an additional 2-3 minutes until cooked through.

Once the water is boiling, add the fettuccine and cook according to package instructions, reserving ½ cup of pasta water before draining.

In the same skillet, melt the remaining unsalted butter and sauté the garlic and red pepper flakes for about 1 minute. Stir in the heavy cream and whisk in the egg yolk, letting it simmer for 2-3 minutes until slightly thickened.

Gradually add the drained fettuccine to the sauce, mixing thoroughly. Incorporate the lemon juice, parmesan cheese, and ground nutmeg. Adjust the sauce consistency with reserved pasta water.

Finally, toss in the cooked chicken and baby spinach, cooking for an additional 2 minutes. Adjust seasoning with more salt or pepper as needed.