Preheat oven to 400°F. In a medium bowl, whisk together flour, walnuts, 2 Tbsp. of sugar, brown sugar, cinnamon, nutmeg and a pinch of salt. Pour butter over dry ingredients and mix with a fork, breaking up any large chunks. Set aside.
In a separate bowl, combine apple cubes with remaining 2 Tbsp. sugar and lemon juice, tossing to coat. Divide among four small oven-safe ramekins and top each with 1/4 of walnut mixture.
Place ramekins on a baking sheet and bake until fruit is soft and topping is browned, 30 to 35 minutes. Serve warm with scoops of vanilla ice cream, if desired.