Bring a large pot of salted water to a boil over high heat and cook the pasta according to the package instructions. Reserve ¾ cup (175 ml) of pasta cooking water, then drain the pasta in a colander and place the pot back on the stove over medium heat
To the pot, add the olive oil, onion, red pepper flakes, and garlic. Cook until fragrant, about 2 minutes. Add the tomatoes and reserved pasta water, and simmer until reduced by half, about 4 minutes. Season with the salt and pepper.
Add the pasta back to the pot, along with the shredded chicken and spinach, and stir to combine and wilt the spinach.
Store the leftovers in the fridge for the rest of the week.
*Vegan/Vegetarian Substitution: Omit the shredded chicken.