Preheat oven to 375 degrees F.
Use one tablespoon of the butter to grease the bottom and sides of a 9-inch square baking pan, and sprinkle one tablespoon flour onto pan, shake to coat. Spread sliced apples evenly in pan.
In a bowl, mix the oats, flour, sugar and cinnamon, and nuts and salt if using. Stir so ingredients are mixed evenly.
Add the butter mix into dry ingredients—use clean hands and fingers or a spoon or spatula. Mix gently until oats are coated—it will be clumpy and stiff.
Spoon or drop the oat mixture across the top of the apples so they are loosely but evenly covered.
Bake for 30 min., or until the topping is browned and the apples are soft—test with a fork, and bake 10-15 min. longer if the apples are still firm.
Serve warm, with a spoonful of ice cream or whipped cream for an extra treat!