Preheat oven to 375 degrees F. Rub butter in a 7"x11" baking dish (or similar size) and set aside.
To Make the Topping
Combine the flour, oats, pecans, brown sugar, baking powder, cinnamon and salt in a large bowl and whisk together. Add in the chilled butter and use clean fingers to pinch the butter and create a coarse mixture. Place bowl in fridge until ready to use.
To Make the Filling
In a large bowl whisk together ¼ cup melted butter with 2 tbsp flour. Add in the water, lemon juice, vanilla, brown sugar, cinnamon and pinch of salt and whisk to combine.
Stir in the sliced peaches and coat evenly with the brown sugar mixture. Pour peach mixture into the prepared baking dish and top with the oat crumble topping.
Bake for 35 to 45 minutes, or until center is bubbly and top is browned nicely. Let the peach crisp cool for at least 5 minutes before serving.
Notes
*Keep peach crisp covered and store at room temperature. **Can be prepped ahead of time, covered and stored in refrigerator prior to baking up to 24 hours. If baking from chilled, you'll need to bake longer (until center is bubbly).