Adapted from Smitten Kitchen
Make crust: Mix flour and salt together in a large bowl and work butter into the flour with a pastry blender or by hand until dough is crumbly then add cold water until it forms a soft ball.
Roll your dough out into a 1/4" sheet and place in a 9" tart pan, trim the edges then chill in fridge while you make the onions.
Make filling: Melt caramelized onions by placing the butter and olive oil in a deep pan over medium heat.
Once completely heated, add the onions and cook for 15 minutes.
Add the salt and sugar and continue cooking the onions for another 15 minutes until they brown through completely.
Once brown, add the stock to deglaze the pan, turn the heat to high and simmer for 10 minutes or until the liquid evaporates. Let cool completely.
Once cool add the egg and cream to the onions, then add the gruyere and pour that mixture into the tart pan.
Heat oven to 200°C and bake for 25 to 30 minutes until egg mixture is set.
Servings 0