To prepare the dough, mix together the flour, garlic powder, salt (if using), and pepper. Cut the butter into chunks, then add to flour mixture. Use the palm of your hands to rub the flour and butter together until butter pieces are no larger than pea-sized.
Mix together cooking wine, red wine vinegar, and water, then drizzle, tablespoon by tablespoon, over the dough. Mix gently just until a shaggy dough comes together, adding more liquid if the dough is too dry. If you pinch the dough together and it mostly stays put, do not add more liquid! The dough should easily come together into a ball without being too sticky or mushy.
Gather the dough into a ball. Place on a sheet of plastic wrap and press into a disc about one inch thick. Cover with plastic wrap and let chill in the fridge while you prepare the filling.
In a large skillet over medium-high heat, cook the onions in the oil. Add the salt and pepper. Once the onions have started to soften, add the mushrooms, then the ham (if using). Lower the heat to medium and cook, stirring occasionally, until mushrooms start to soften, about 4-5 minutes.
Add the garlic, then the spinach. When the spinach starts to wilt, add the red wine vinegar, then continue to cook until spinach is completely wilted.
Preheat oven to 400F.
Remove the dough from the fridge and whack it a few times with the rolling pin to flatten out more. Roll out the dough on a lightly floured surface to form as close to a circle as you can (don't worry if it's not that close. Just call it rustic). The dough should be between 1/8-1/4" thick.
Transfer dough onto a parchment-lined or aluminum-foil lined baking sheet (if using aluminum foil, *lightly* dust it with flour).
Spread the jam into the middle of the dough circle, leaving about 1 1/2 to 2 inches from the edge of the dough. Sprinkle with half of the Parmesan cheese.
Add the spinach filling, again leaving 1 1/2 to 2 inches from the edge of the dough. Arrange the sliced pears on top in an aesthetic way by fanning them out, then sprinkle the rest of the cheese on top.
Fold the edges over, then brush the dough with the cream, milk, oil, or melted butter.
Bake for 30-40 minutes, or until filling is bubbly and the crust is golden brown.
When done baking, transfer to wire rack and let cool for ten minutes before slicing and eating, or let cool completely before transferring to a plate and covering with plastic wrap to refrigerate.