Preheat oven to 350°F (175°C) and lightly grease a 9-inch square baking dish.
In a large bowl, toss sliced apples with lemon juice, 1 teaspoon of cinnamon, and 2 tablespoons of maple syrup.
Arrange the apple mixture in an even layer within the prepared baking dish.
In a separate bowl, combine oats, almond flour, chopped walnuts, melted coconut oil, remaining 3 tablespoons of maple syrup, remaining 1/2 teaspoon of cinnamon, and sea salt.
Mix the topping ingredients until a crumbly texture forms, then distribute evenly over the apples.
Bake for 40 to 45 minutes until the apples are soft and the oat topping is golden brown.
Allow the crisp to cool for 10 minutes to set before serving.