Add the medjool dates, water, and brown sugar to a saucepan over medium high heat. Bring to a boil then reduce heat to simmer. Stir the mixture constantly so that it doesn't burn (sugar burns easily). You'll know it's done when the dates have broken down into a thickened jam-like consistency.
Remove the saucepan from the heat source, then stir in lemon juice. Set aside.
In a bowl, mix the large flake oats, all purpose flour, brown sugar, baking powder and salt.
Add in cold butter pieces and use a pastry cutter to create a crumble-like texture of the butter. I like to use my hands at this point to break up any large pieces of butter still left.
Pour 50-60% of the oat mixture into a square baking pan making sure to cover completely. Press oats down firmly with your hands. Spoon the date mixture overtop, and gently spread with a spatula (Focus on gently spreading the top of the date mixture around with spatula to avoid disrupting the oat base). Sprinkle the rest of the oat flour mixture on top and spread evenly. Lightly press it into the pan.