Combine the butter and vanilla pod half and seeds in a small saucepan. Melt the butter over medium heat, and continue to cook until the butter turns golden with dark brown flecks, about 5 minutes, swirling frequently. Toward the end, the butter will foam up dramatically; watch it closely at this point. Let the browned butter cool at room temperature while you prepare the filling. (If the butter is at risk of over-browning, pour it into a heatproof bowl or measuring cup to stop the cooking.) Remove the pod and discard (or wash, dry, and save it for another use).
Prepare the filling:
Position a rack in the upper third of the oven and preheat to 375º. Have a 10" oven-proof skillet (or a 10" solid tart or pie pan, or something equivalenat the ready
Cut the peaches in half, then cut each half into six to eight slices. Place in a large bowl (or right into the baking vesseand sprinkle with the bourbon.
In a small bowl, combine the maple sugar and vanilla pod and scrapings, rubbing the seeds into the sugar to distribute them evenly. Remove and discard the pod (or save it for another use). Stir the cornstarch into the sugar, then add to the bourbony peaches, tossing gently to coat. Distribute the peaches evenly in the baking vessel.
Make the topping:
In a large bowl, combine the flour, oats, maple sugar, nutmeg and salt. Stir in the melted, cooled brown butter, stirring with a spoon or your hands until the mixture forms large clumps.
Scatter the crumble topping over the peaches, taking the time to form some of it into small, hazelnut-sized balls. Scatter a bit of flaky salt over the top, as well as some coarse sugar.
Bake the crisp until the topping is golden and the fruit is bubbling, 45-60 minutes. Let cool at least 20 minutes. Serve warm or at room temperature, preferably with vanilla ice cream. (Leftovers make a tasty breakfast, cold from the fridge, with yogurt instead of ice cream.)
If you like nuts in your crisp, I think this topping would be tasty with 1/4 cup of chopped, toasted pecans.If you don't have a vanilla bean on hand, add 1/2 teaspoon of vanilla extract to the cooled, browned butter, and another 1/2 teaspoon to the filling along with the bourbon. I used Bulleit bourbon here, but any brand will do.If you don't have or can't find maple sugar, substitute an equal amount of light or dark brown sugar, or an unrefined sugar such as Alter Eco, in the topping, and maple syrup in the filling.And if you only have old-fashioned rolled oats, David Leite says you can whiz them in a coffee grinder to break them up a bit.Nutritional values are based on one of eight servings.