Finely grind biscuits and 1 tsp of cinnamon in blender. Add butter and process until combined. Press mixture into base of a lined, buttered, 23cm springform pan. Chill in the tin for 2 hrs. Fill the tart case with the thick custard, smooth the surface and chill for a further 30 mins.
Preheat the oven to 200°C and chop the trimmed rhubarb stalks into 5cm lengths and toss through the brown sugar and remaining cinnamon in a baking dish. Squeeze half a lemon over the rhubarb and cover the dish with foil and bake for 30 mins. Allow to cool.
Carefully remove tart from the tin and top with the rhubarb compote.