Bring a large pot of salted water to a boil and cook the spaghetti according to package directions until al dente. Before draining, scoop out and reserve ¼ cup pasta water. Drain the rest, add 1 tablespoon olive oil to the pasta, toss, and set aside.

Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add the diced onion and sauté until soft and translucent, about 4–5 minutes.

Add minced garlic and cook for 30–60 seconds, stirring, until fragrant. Don’t let it brown.

Add the remaining olive oil and the halved cherry tomatoes. Sauté for several minutes until the tomatoes soften and begin to release their juices. You want them collapsing, not pureed.

Stir in the baby spinach and cook until wilted, 1–2 minutes.

Add the sliced kalamata olives, lemon juice, red wine vinegar, dried oregano, salt, and pepper. Cook together for another minute to marry the flavors.

Remove the skillet from heat. Add the cooked spaghetti and toss to coat in the tomato-spinach mixture. If the pasta seems dry, add the reserved ¼ cup pasta water a little at a time until the sauce clings to the noodles.

Add the crumbled feta and the thinly sliced basil. Toss gently one more time to distribute the cheese without breaking it down completely.

Garnish with extra basil or parsley if desired and serve immediately.