4 slices bacon, cut in half crosswise, ½ cup water
Line a large plate with paper towels and set it next to the stove. Lay the bacon strips down in a cold 12-inch skillet. Wash your hands. Pour the water over the bacon in the skillet.
Cook over high heat until the water comes to a boil (big bubbles will vigorously break all over the surface). Reduce the heat to medium and continue cooking until the water evaporates, 5 to 8 minutes.
Reduce the heat to medium-low. Continue cooking, using tongs to flip the bacon slices over occasionally, until both sides are evenly browned and the bacon is crisp, 8 to 10 minutes. (The bacon should be sizzling gently, but the fat may still occasionally pop, so stand back from the pan and be careful when flipping.)
Turn off the stove and use oven mitts to carefully slide the skillet to a cool burner (cast iron skillet handles can get extra hot during cooking; this step is best done by an adult).
Use the tongs to carefully transfer the cooked bacon to the paper towel-lined plate to cool. Let the bacon fat cool slightly, at least 2 minutes.
1 large or 2 small yellow onions, peeled and sliced
Carefully add the sliced onions to the bacon fat left in the skillet (the fat will still be hot, so be careful of splashes). Return the skillet to medium heat and cook, stirring occasionally with a wooden spoon, until the onions are softened and beginning to brown, 3 to 4 minutes.
3 tart apples, cored and sliced into ¼-inch-thick wedges, 1 tablespoon packed brown sugar, ¼ teaspoon table salt
Cover the onions with the apple slices in an even layer. Sprinkle the brown sugar and salt evenly over the top of the apples. Place a lid on the skillet and cook until the apples are tender, about 5 minutes.
While the apples and onions are cooking, crumble the bacon slices into small pieces.
When the apples are ready, use an oven mitt to remove the lid from the skillet. Stir the mixture with the wooden spoon until the apples and onions are evenly coated with sauce, then turn off the stove. Sprinkle the crumbled bacon over the top and serve.