Slice the apples into uniform, thin slices using a chef's knife on a cutting board, ensuring all pieces are similar in size for even cooking.
Measure out the sugar and cinnamon, then add them to a medium saucepan alongside the sliced apples. Toss gently to distribute the spices evenly over the apples.
Bring the mixture to medium heat and cook, stirring occasionally with a wooden spoon or spatula, until the apples start to release juices and become tender, about 8-10 minutes. The mixture should look glossy and simmer gently.
Add the small pieces of butter to the saucepan. Continue cooking and stirring until the butter melts completely and the mixture thickens slightly, approximately 3-5 minutes. You should see a shiny, caramelized coating forming around the apples.
Once bubbling and thickened, remove the saucepan from heat. The filling should be tender, glossy, and coated evenly, ready to be poured into a pie crust or stored for later use.
Transfer the hot filling into a prepared pie crust or container for cooling and use in your favorite baking recipes. Optionally, garnish with a sprinkle of cinnamon or a drizzle of caramel for presentation.