Set out two clean 16-ounce jars and one small covered container for the crisp apple topping.
Core the washed apple. Cut half into 1/4-inch dice and half into 3/8-inch topping pieces. Toss with lemon juice; reserve the larger pieces in the small container and divide fine dice between jars.
Whisk milk, Greek yogurt, maple syrup, vanilla, cinnamon and salt in a medium bowl until smooth.
Stir in rolled oats and chia until every oat is coated, then divide evenly between the two jars and distribute the fine apple dice.
Rest open jars for 10 minutes, then stir thoroughly from the bottom to redistribute settling chia and liquid.
Cover the jars and apple topping. Refrigerate at 40°F or below for at least 6 hours and up to overnight.
Stir each jar in the morning. Add 1 tablespoon milk for a looser texture; if liquid pools, stir and wait 2 minutes before correcting.
Top with the separately chilled apple pieces and optional walnuts immediately before serving.
