We made about 6 different versions of this with our haul of fall apples. The recipe can be edited to one’s taste (and should be, as everyone in the family liked it slightly differently).
1 tbsp lemon juice
2 tsp cinnamon
0.5 cup flour
1 cup oats
1/3 cup sugar
6 tbsp butter
Slice the apples and toss with the lemon juice and the cinnamon. You may wish to add a bit of sugar here, depending on the sweetness of your apples. Maybe a tiny bit of water is also necessary, if your apples are dry or not very ripe.
Mix the flour with the sugar and oats. Add in the butter melted, and mix it well. Top the apples with this mix. Add a bit of water, or more butter if it seems dry (water makes it more cakey/oatmeal-like, butter makes it more crispy).
Bake in a preheated 375F oven for 30-45 minutes, to desired color.