Bring a large pot of salted water to a boil. Add pasta and cook until al dente. Prior to draining, remove 1 cup of pasta cooking water; set aside. Drain pasta, and add back to pot.
Meanwhile, heat olive oil in a large skillet over medium-high heat. Once hot, add tomatoes and onion; cook 6 to 8 minutes, until the tomatoes burst and onion softens. Add garlic, oregano, fennel seeds, and red pepper flakes; cook 2 minutes, until aromatic. Add spinach; cook 1 to 2 minutes, stirring constantly, until wilted. Stir in lemon juice, rosemary, salt, and pepper.
Pour tomato mixture into pot with cooked pasta, and slowly stream in 1 cup reserved pasta cooking water, tossing constantly with a set of tongs. Continue tossing until a light sauce comes together and clings to the pasta. Stir in feta cheese and olives.Divide pasta evenly into 6 bowls, and top with additional herbs and/or cheese, if desired.
Enjoy pasta as is, or add seared shrimp or scallops, canned flaked tuna, chickpeas, or shredded rotisserie chicken for extra protein. Store: Transfer leftovers to an airtight container and refrigerate for up to 4 days. Reheat: Warm gently in the microwave with a splash of water or broth to keep the pasta from drying out.