Bring a large pot of water to a boil and season the chicken with salt and black pepper.
Heat the olive oil in a large skillet and cook the chicken for 3 to 4 minutes per side, until it reaches 165°F, then rest and slice it.
Boil the spaghetti until al dente, reserving 1 cup pasta water before draining.
Whisk the ricotta, lemon zest, lemon juice, Parmesan and 1/2 cup hot pasta water in the warm pot.
Fold in the spaghetti and spinach, add enough pasta water to loosen the sauce and top with the sliced chicken.