Mix the flour, sugar and salt together. Cut in the butter or margarine with a pastry cutter or food processor until evenly distributed. Reserving a cup and a half of the mixture, press the remaining in to a foil-lined 9x13" pan.

Bake on 350 F for 20 minutes.

Mix the oats and brown sugar in to the reserved crumb mixture and set aside.

Stir together the flour and sugar, the whisk in the eggs until smooth. Add in the jam, lemon juice and lemon zest and continue whisking until combined.

Pour over the crust, and bake for 15 minutes. Then top with the remaining crumbs, and bake another 20 minutes. Let cool and then chill in the refrigerator before cutting. Store in the fridge.