Step 1Heat the oven to 375 degrees. Quarter, core and peel the pears. Spinkling the pears with lemon juice as you go to prevent browning, cut each quarter crosswise into one-fourth-inch-thick slices. You should have about 6 cups of sliced pears.
Step 2Once you have sliced all the pears, add 2 tablespoons of the brown sugar and 2 tablespoons of the flour to the pears, tossing to coat. With the tip of a knife, scrape the seeds from the vanilla bean onto the pears and toss to coat evenly. Set them aside while you prepare the topping.
Step 3Combine the remaining three-fourths cup brown sugar, 1 cup flour, baking powder, ground ginger, kosher salt and rosemary or thyme. Add the butter and work into the dry ingredients until it is crumbly.
Step 4Spoon the pear mixture into a 9-inch square baking dish. Sprinkle crumb topping evenly over the top of the fruit. Bake 35 to 40 minutes, until bubbly and lightly browned on top. Remove from the oven and let stand until warm before serving.