Peel each pear with a vegetable peeler, then cut them in half and scoop out the core with a spoon or melon baller. Slice each half into thin wedges, about a quarter inch thick, so they cook evenly and soften at the same rate as the custard sets. Arrange the slices in your greased pie dish in a single layer, slightly overlapping if needed.
Add the eggs, milk, heavy cream, sugar, flour, melted butter, vanilla, salt and cinnamon (if using) into a blender or large bowl. Blend or whisk everything together until the mixture is completely smooth with no lumps of flour remaining. This should only take about a minute in a blender, or a couple of minutes of steady whisking by hand.
Pour the custard mixture slowly and evenly over the arranged pear slices in the baking dish. Give the pan a gentle shake so the batter settles into all the gaps between the pears. This step is quick, but pouring slowly helps keep the pears from shifting out of place.
Place the dish in a preheated 350 degree Fahrenheit oven and bake for 40 to 45 minutes. You're looking for a golden top that is firm to the touch with just a slight jiggle in the center when you gently shake the pan. If the top browns too quickly, loosely tent it with foil for the last ten minutes.
Let the custard cool on a wire rack for about 15 to 20 minutes before slicing. It can be served warm, at room temperature, or chilled, depending on your preference. Dust generously with powdered sugar using a fine mesh sieve right before you plan to serve it.