Preheat the oven to 350F
If you can’t find oat flour, just put some rolled oats into a blender and give it a few pulses until the flakes become flour. You may have to stop and move the oats around a couple of times to ensure you are grinding every flake into flour.
Melt the butter. I prefer to do this in a small saucepan on the stovetop. It has happened too often that I put it into the microwave, and it explodes all over. So to make things easier, I do it the old fashion way. If you want to use the microwave, I suggest you do it slowly and don’t put it in for more than 15 seconds at a time.
In a small bowl, mix the oat flour, almond flour, brown sugar, and a pinch of salt. Mix them, and then add the melted butter. Give all this a nice mix until the mixture resembles sand. Too much butter will result in a soggy crumble, and too little may give you a tough top layer.
Squeeze half a lemon into a 9x13 baking dish. Core and slice the apples into wedges and add them to the baking dish as well. Mix them with the lemon juice after adding wedges from each apple. When all apples are in, mix in the cinnamon and ginger. Give all this a toss to coat evenly with the lemon juice and spices.
Top the apples evenly with the sugar/flour crumb mixture and place the baking dish, uncovered, into the preheated oven. Bake for 50 to 60 mins or until the top is brown and crunchy.