Bring a large pot of salted water to a boil. Cook spaghetti according to package directions until al dente. Drain, reserving 1 cup of pasta water.
Meanwhile, heat olive oil in a large skillet over medium-high heat. Season chicken pieces with salt and pepper, then cook until golden and cooked through, about 6-8 minutes. Remove chicken from skillet and set aside.
In the same skillet, add onion and cook until softened, about 4 minutes. Add garlic and cook for 1 minute more.
Stir in crushed tomatoes, oregano, and basil. Bring to a simmer and cook for 10 minutes, stirring occasionally.
Return chicken to the sauce, add cooked spaghetti, and toss to combine. Add some reserved pasta water if needed to loosen the sauce.
Serve hot, garnished with fresh basil and grated Parmesan cheese.
Tips
For a richer flavor, use a splash of white wine when cooking the onions.
Leftovers can be stored in an airtight container in the refrigerator for up to 3 days.
Reheat gently on the stovetop with a splash of water or broth.