Preheat oven to 400°F (204°C).
Brush the cut sides of the apples with 1 teaspoon lemon juice. Arrange the apples cut-side up in a baking dish.
4 large Granny Smith apples , 2 tsps (10 ml) fresh lemon juice
Stir together the brown sugar and cinnamon in a small bowl, then sprinkle the mixture evenly over the cut sides of the apples. Pour the maple syrup into the bottom of the baking dish around the apples.
2 tbsps (25 g) packed light brown sugar, ¼ tsp (0.5 g) ground cinnamon, 1 cup (315 g) maple syrup
Cover the baking dish with foil and bake for 10 minutes. Remove the foil and continue baking for about 15 minutes, spooning some of the maple cooking liquid over the apples as they bake. The apples are ready when a paring knife slides easily into the flesh but the halves are still firm enough to transfer to a serving platter without breaking apart. Larger apples may require additional baking time.
Transfer the baked apples to a serving platter. Carefully pour all of the maple syrup and apple juices from the baking dish into a small saucepan.
Add the remaining 1 teaspoon lemon juice and the salt to the saucepan and bring the mixture to a boil. Cook for 3 to 5 minutes, stirring frequently, until the maple-apple sauce has reduced and is slightly thicker. The amount of liquid released by the apples can vary, so use the consistency of the sauce as your guide instead of relying only on the cooking time.
⅛ tsp (0.75 g) table salt
Remove the sauce from the heat and stir in the toasted walnuts. Spoon the warm maple-walnut sauce over the baked apples.
½ cup (60 g) chopped walnuts
Serve warm, with vanilla ice cream or whipped cream if desired.
vanilla ice cream, whipped cream