Preheat your oven to 375°F (190°C). Grease a baking dish or cake pan with cooking spray or butter.
In a mixing bowl, combine the cooked spinach, mashed butternut squash, grated Parmesan cheese, bread crumbs, heavy cream or milk, minced garlic, salt, and black pepper. Mix well until all the ingredients are thoroughly combined.
In a separate bowl, whisk the eggs until well beaten. Add the beaten eggs to the spinach and butternut squash mixture. Stir until everything is evenly incorporated.
Transfer the mixture to the greased baking dish or cake pan, spreading it out evenly.
Bake the creamed spinach and butternut cake in the preheated oven for 30-35 minutes, or until the top is golden brown and the cake is set.
Remove from the oven and let it cool for a few minutes before serving.
Slice the cake into squares or wedges and serve warm as a side dish or a standalone meal. Garnish with additional grated Parmesan cheese or fresh herbs, if desired.