Preheat the oven to 350°F (175°C). Line a baking sheet with parchment or a silicone mat.

In a large bowl, cream the softened butter with the granulated and brown sugars until smooth and slightly fluffy.

Beat in the egg and vanilla until fully incorporated.

In a separate bowl, whisk together the flour, salt, cinnamon, and oats. Gradually add the dry mix to the wet ingredients and stir until just combined.

Fold in the diced apples and lemon juice. Stir in nuts if using. If the dough seems very sticky from the apples, add 1–2 tablespoons more flour.

Drop rounded tablespoons of dough or use a small cookie scoop to place dough 2 inches apart on the prepared sheet. Press gently to flatten slightly if you prefer a more even bake.

Bake for 12–15 minutes, or until the edges are set and the tops turn a light golden color. The centers will be soft but should not look raw.

Allow cookies to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.