Fill a large pot halfway with well-salted water and bring to a boil. 

Whisk together the cream and egg yolk and set aside.

In a deep 12-inch frying pan, melt one tablespoon of the butter over medium-high heat.

Cut chicken into bite-sized pieces. (Try using kitchen shears to avoid dirtying a cutting board.)

Place chicken into pan in a single layer and sprinkle generously with salt and pepper.

Cook until lightly browned on the underside and opaque halfway through, about three minutes. Flip once and cook until just cooked through. Remove to a plate and set aside.

When water boils, cook fettuccine until one minute shy of al dente according to package directions. Before draining pasta, scoop out ½ cup of the cooking water and set aside. 

Meanwhile, melt remaining tablespoon of butter over medium-low heat in the same frying pan. Add garlic and cook for one minute, stirring constantly.

Pour in cream mixture. Simmer gently, adjusting heat if necessary to prevent boiling, while pasta cooks.

When pasta is ready, stir lemon juice and cheese into sauce until smooth and creamy. Whisk in reserved pasta water until smooth.

Add pasta and spinach, raise heat to medium, and toss with tongs for a couple of minutes until spinach is wilted and sauce clings nicely to pasta.

Stir in chicken along with any accumulated juices.

Off the heat, season with freshly ground black pepper. 

Divide among four shallow bowls and serve immediately.