Bring a large pot of salted water to a boil and cook pasta until al dente.
Season chicken cubes with salt and pepper.
In a large skillet, heat olive oil over medium-high heat and sear chicken until cooked through, approximately 6 to 8 minutes.
Remove chicken from skillet and set aside; add minced garlic and diced shallot to the same skillet, sautéing for 2 minutes until translucent.
Reduce heat to low and stir in the ricotta cheese and 1/4 cup of reserved pasta water until a smooth sauce forms.
Add the cooked pasta, chicken, and baby spinach to the skillet.
Toss ingredients together, adding more pasta water if necessary to achieve a velvety consistency.
Stir in Parmesan cheese, lemon zest, and red pepper flakes.
Adjust seasoning with salt and pepper and serve immediately.