For the crust: combine all ingredients in a Tupperware container. Close the lid and shake it vigorously until dough comes together. Press the ball of dough in a tart pan lined with baking paper. Pierce with a fork
For the filling: heat a small amount of oil in a wide pan over medium low heat. Add the onions and cook for around 20min, until jammy but not browned! Season to taste, add sugar, vinegar and butter
Pour the filling into the crust and bake in a 180C/350F oven for 30min or until onions are caramelized and the crust is a bit golden. I bake this tart on the lower oven rack to prevent onions from burning
