Preheat oven to 350°F. Lightly spray a 9-inch round pie pan with cooking spray or line it with parchment paper. Set aside.
If you will be using the streusel topping, make it now (instructions below) and set aside. If you are not using the streusel topping, skip to the next step.
In a medium mixing bowl, whisk together the flour, baking powder, baking soda, cinnamon, ginger, nutmeg, and salt.
In a large mixing bowl, whisk together the yogurt and sugar until well combined. Whisk in the milk, lemon juice, and vanilla.
Stir the dry ingredients into the wet ingredients until just combined. Do not over mix! No white flour should be visible, but lumps are okay.
Gently stir in the diced pear.
Pour the batter into the prepared cake pan, making sure to scrape down the sides of the bowl to get it all in the pan. Smooth out the top with a spatula.
Arrange the pear slices on top of the batter either fanned out in a circle or however you think looks pretty.
Sprinkle the streusel, if using, evenly over the top of the cake and pear slices.
Bake for 35 to 40 minutes until the cake starts to pull away from the sides of the pan and a toothpick inserted into the center of the cake comes out clean.
Transfer the cake, still in the pan, to a wire cooling rack to cool for about 15 to 20 minutes before slicing.