Arrange the butternut squash and onions on a shallow baking sheet. Season with salt and pepper and toss with the olive oil.
Bake until the squash is tender and lightly browned, 20 to 30 minutes.
Remove from the oven and add spinach. Mix to combine. Return the baking sheet to the oven and turn the oven off.
Meanwhile, in a large bowl, combine ricotta, parmesan, mozzarella, cranberries, eggs, Italian seasoning, salt, and pepper.
Generously grease a 10-inch bundt pan with some of the melted butter. Place 2 sheets of fillo on top of the bundt pan. Poke a hole into the fillo where the center tube is and gently push fillo into the pan to line it. Continue fitting fillo sheets into the pan, turning it as you go in order to make even layers (the sheets will overlap in the center hole of the pan) until all sheets are used. Edges of fillo should hang over the edges of the pan.
Spoon half of the ricotta filling into pan. Spread the roasted butternut squash and spinach over ricotta, then spoon the rest of the ricotta mixture on top. Fold edges of fillo over the filling.
Using a sharp knife, poke many holes in the dough that reach all the way to the bottom of the pan. Slowly pour melted butter over the pie (some butter will remain on top of the dough).
Place pan on a baking sheet and bake at 400 degrees Fahrenheit for 1 - 1 ½ hours, or until pie is puffy and golden brown. If the top is getting brown too quickly, cover with a piece of aluminum foil.
Allow pie to cool in pan. Place a wire rack on top of the pie and invert. Transfer to a serving plate. Serve warm or cold.
Add ham or prosciutto to the pie or instead of butternut squash and spinach, if you prefer.
Don't worry if the fillo sheets tear during the process. It won't' be visible when the pie is baked.
Please keep in mind that nutritional information is a rough estimate and can vary significantly based on its products.