Preheat oven to 425 degrees. Place your oven rack in the center of the oven for even baking.
Peel and core the apples. Cut into 1/4 inch thick slices.
Place the sliced apples on a 18x13 inch sheet pan and toss with the lemon juice, 1/4 c brown sugar, 2 tbsp melted butter, 1 tsp cinnamon, and 1/8 tsp salt together.
Spread the apples evenly on the pan after you have added ingredients. Cover with foil and bake in the oven until the apples have softened and release some of the liquid. ~20min.
While apples are baking, combined the oats, flour, remaining 1.25 c brown sugar, 1 tsp cinnamon and 1 tsp salt together in a large bowl.
Use a fork to break down any of the lumps. Add the remaining melted butter to the oat mixture, stirring with the fork until moist and crumbly. This is when I add the raisins and chocolate chips to the mixture, as desired.
Pull apples from the oven and carefully remove the tin foil. Scatter the oat cumble mixture evenly over the baked apples.
Bake uncovered until the top is crispy and golden brown, juices are bubbly for about 15 minutes.
Serve with ice cream of your choice.