Preheat the oven to 400 degrees F.Butter or oil an 8×12-inch baking dish, or two smaller gratin dishes.Sprinkle with lemon zest and set aside.
Bring a large pot of water to a boil, add salt and cook the penne until al dente.
10 seconds before the pasta is cooked, stir in the spinach and butternut squash; drain quickly and carefully.
Run cold water over the pasta, squash and spinach (just enough to stop it from cooking), shake off any extra water and set aside.
While waiting for the pasta water to boil, whisk together the eggs, yogurt, garlic, and salt in a large mixing bowl. Set aside until the pasta is ready, then add the pasta-squash-spinach mixture to the yogurt mixture and stir in half of the almonds.
Scoop the mixture into the prepared baking dish, sprinkle with Feta cheese and the olive pieces and bake for about 25-30 minutes.
Remove from the oven and serve sprinkled with the remaining almonds and some chopped fresh mint.