Set oven rack at center. Preheat oven to 350F and grease a 9”x9” square cake or glass casserole pan. Line a sheet pan with parchment paper or foil.
Slice apples about ¼” thick and place them in a large mixing bowl. (I used my food processor slicing disc after coring the apples.) In a small cup, whisk together the melted butter and peanut butter. Toss the apples with the peanut butter mixture and with the remaining filling ingredients. Spread the apple mixture in the prepared 9”x9” pan.
To make the topping, whisk (or pulse if using a food processor) together the flour, oats, salt, sugar, cinnamon, and baking powder.
Add the cold butter, working it in using your fingers, a pastry cutter (or pulse slowly and carefully using a food processor) until you have an unevenly crumbly mixture.
Spread the topping evenly over the apples in the pan and sprinkle the chopped peanuts over the topping.
Place the baking pan on the lined sheet pan and bake about an hour until it’s bubbly, the top is golden brown, and the apples are tender when pierced with a paring knife.
Remove from oven; cool at least 20 minutes before serving. It will be soft after 20 minutes but will firm up if it’s cooled completely. Serve as is or with a scoop of vanilla ice cream and a maybe a drizzle of caramel sauce. Store 2 days well-wrapped at room temp, 4-5 days in the fridge, and up to 6 months double wrapped in the freezer.