Preheat the oven: Preheat your oven to 375°F and lightly grease a baking dish large enough to hold the peach halves in a single layer.
Halve the peaches: Cut each peach in half and remove the pit, then arrange the halves cut side up in the prepared baking dish.
Mix the topping: In a small bowl, whisk together the melted butter, brown sugar, cinnamon, lemon juice, and vanilla extract until smooth.
Spoon over peaches: Spoon the butter mixture evenly over each peach half, letting some pool in the center where the pit was.
Add oats and pecans: Sprinkle the oats and chopped pecans over the top of each peach so they settle into the butter mixture.
Bake until soft: Bake for 20-25 minutes, until the peaches are soft and the juices bubble around the edges of the dish.
Broil to brown: Switch the oven to broil and cook for 1-2 minutes, watching closely, until the topping turns golden brown.
Serve warm: Let the peaches cool for a few minutes, then spoon the syrup from the pan over each half and add a scoop of vanilla ice cream if you like.