Place brown sugar, soda, salt, baking flour, oats and xanthan gum in a large bowl; stir to combine. Pour in melted butter and stir until crumbly, moist mixture forms. Reserve a heaping 1/2 cup of mixture and set aside.
Press remaining crumb mixture into the bottom of a parchment-lined 13 x 9-inch baking pan. Spread jam to within 1/4" of the edge. Sprinkle the reserved 1/2 cup of crumb mixture evenly on top and press it down lightly into jam.
Bake at 350°F for 30 minutes or until lightly browned and bubbly. Allow to cool completely before cutting into bars. Makes 24 bars.
