Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. The pasta should have a slight bite to it since it will continue to absorb flavors as it sits. Drain and rinse with cold water to stop the cooking process and cool it quickly.

While the pasta cooks, wash and roughly chop the baby spinach into bite-sized pieces. Halve the cherry tomatoes and slice the red onion as thinly as possible.

Heat a small skillet over medium heat and add the pine nuts. Toast them for 2-3 minutes, stirring frequently, until they're golden brown and fragrant. Watch them carefully as they can burn quickly.

In a small bowl, whisk together olive oil, lemon juice, minced garlic, and oregano. Season with salt and pepper. The dressing should taste bright and slightly tangy.

In a large serving bowl, combine the cooled pasta, chopped spinach, tomatoes, red onion, and toasted pine nuts. Pour the dressing over the mixture and toss gently until everything is well coated. Add the crumbled feta cheese last and fold it in carefully to prevent it from breaking apart completely.