Bring a large pot of salted water to a rolling boil. Add the rigatoni and cook until al dente, about 10-12 minutes. Drain and rinse under cold water.

In a small bowl, combine lemon juice, olive oil, salt, and pepper. Whisk vigorously for about 30 seconds until emulsified.

In a large mixing bowl, add the cooled rigatoni, feta, cranberries, walnuts, spinach, and onion. Gently fold to combine.

Drizzle the lemon vinaigrette over the salad and toss carefully to coat all ingredients.

Serve immediately or chill in the refrigerator for about 15 minutes before serving.