Preheat oven to 350 degrees F. Line a 9 inch square metal pan with non-stick foil or line with parchment paper.
In a bowl or food processor, combine brown sugar, oats, flour, baking soda and salt. Cut in or process in 9 tablespoons of butter until mixture is coarse and crumbly. Reserve about ¾ cup for topping. Pat remaining into bottom of pan. It will seem dry, but it’s supposed to be.
Toss apples with half of the lemon juice and arrange in a fairly layer across crust. Sprinkle with cinnamon.
In a saucepan, combine sugar, cornstarch and water. Turn heat to medium and cook, whisking constantly, until mixture begins to boil and thicken. Remove from heat and stir in vanilla, remaining 1 tablespoon butter and remaining teaspoon of lemon juice. Pour over apples.
Sprinkle reserved crumbs on top, then sprinkle pecans over crumbs. Bake at 350 degrees F for 35-45 minutes or until top is lightly browned.
If you’d prefer bars that hold together, let the bars cool completely and then chill before cutting. You can bring them back to room temp after chilling, of course.