Preheat oven to 425 degrees F. From lemon, grate 1/2 teaspoon peel and squeeze 2 tablespoons juice. In 13- by 9-inch glass or ceramic baking dish, toss lemon peel and juice with apple wedges, 1/3 cup brown sugar, 2 tablespoons flour, ground cinnamon, and salt until apples are evenly coated.
In medium bowl, mix oats with remaining 1/4 cup sugar and 1/3 cup flour. With fingertips, blend in butter until mixture resembles coarse crumbs. Press crumb mixture into clumps and sprinkle over apple mixture.
Bake apple crisp 30 to 35 minutes or until apples are tender and topping is lightly browned. Cool crisp on wire rack for 10 minutes to serve warm, or cool completely (1 hour) on rack to serve later. Reheat if desired.