1. Preheat your oven to 375 and grease a 9 x 13 pan with butter or baking spray.
2. In a large bowl combine all of your filling ingredients together. Gently fold together so that all of the wet and dry ingredients mingle together. Let stand for 10 minutes or so. This will give the pears time to macerate, or pull some of their juices out into the filling prior to baking. 
3. Take the cold butter and cut it in half lengthwise and then again so you have four long, skinny lengths of butter. Line them up side by side and then cut into 8-10 pieces. This will give you thirty to forty cubes of butter. 
4. In a bowl, add brown sugar, oats and salt. Toss them together with your hands to incorporate and then add in the butter. Use your thumbs and fingers to rub the butter into the dry ingredients. Push the butter flat, forcing oats and sugar into the cube, over and over again until all of the butter cubes have been incorporated. Then, take your hand and grab the mixtures as if making a snowball. Do that a few times to make big clumps and then break them up. The idea is to make clusters of oat topping to sprinkle over the filling. 
5. Pour the filling into the prepared 9 x 13 baking dish. Scatter the oat topping over the top, and garnish with flaky sea salt and a bit of coarse sugar if you’d like.

Bake for 30 minutes, or until the oat topping is browned and filling juices are bubbling. Tip: If you like a bit of a toasty flavor, broil for up to 1 minute.