Preheat the oven to 375°. Spray a 2 quart casserole dish with vegetable spray and set aside.
Peel the pears and cut them in half vertically. Use a melon baller to scoop out the seeds and discard. Cut pears into a 1/2" dice and transfer to a large bowl.
Add lemon zest and lemon juice to the pears and toss to coat. Add the brown sugar, chopped thyme, rosemary and salt. Add the slurry and toss again to combine. Stir in the cranberries and set aside.
FOR THE OATMEAL TOPPING:
In a medium bowl, combine the flour, salt, cinnamon, ginger, nutmeg & brown sugar and whisk together.
Make sure the butter is very cold or even frozen. Reserve 1 tablespoon of butter. Use the large holes on a box grater to quickly grate the remaining butter and add it to the flour mixture. Toss to combine. Stir in the oatmeal.
If you don't have a box grater, slice the butter into small cubes and use a pastry cutter to cut it into the flour mixture.
ASSEMBLE THE CRISP:
Transfer the pear and cranberry mixture to the baking dish and spread evenly in the pan. Sprinkle handfuls of the crumble to the pear and spread in an even layer over the fruit. Cut the remaining butter into small pieces and dot them over the top of the crumble.
Bake for 40-45 minutes or until fruit is tender and crisp is golden. Note, if the crisp is browning too quickly, rest a piece of aluminum foil over the top to stop the browning process.
Let the crisp rest for about 10 minutes before serving. Spoon into shallow bowls and serve with vanilla ice cream or whipped cream (optional)
Notes
From wikipedia: Arrowroot makes clear, shimmering fruit gels, is used in cooking to produce a clear, thickened sauce, such as a fruit sauce. It will not make the sauce go cloudy, like cornstarch, flour or other starchy thickening agents would. However, if you don't have arrowroot - you can substitute cornstarch.