For the pastry: Rub together the butter and the flour to reach a breadcrumb-like consistency. Add the egg and mix together until it forms a neat pastry ball.
Wrap in clingfilm and let set in the refrigerator for an hour.
Once rested, take the pastry out and roll flat to approximately ¼ inch thickness, measure
and cut 4 square pieces (7 x 7 inches) to fit over and line a circular pastry cutter (approx. 3 inches in diameter).
Then blind bake each of the tart pastry cases for 12-15 minutes at 180°C.
For the filling: Heat the butter in a large frying pan on the stove. Add the sliced onions and garlic to the pan.
Cook over medium heat until lightly coloured and very soft (about 15-20 minutes).
Remove the mixture from the heat, and continue stirring for a few seconds while adding thyme.
Place the mixture into a bowl and season generously, particularly with black pepper, then set aside to cool.
Once cooled, add 3 egg yolks, milk (or double cream) and Parmigiano-Reggiano cheese.
Gently mix all ingredients together, add black pepper and pour mixture into tart pastry cases.
Bake in the oven at 160°C until the tart is set. Remove and serve with mixed dressed watercress.
Served with a White Bourgogne wine: Chablis or Hautes Côtes de Beaune.
4 large white onions, diced
275ml warmed milk (or double cream)
85g finely grated Parmigiano-Reggiano cheese
3 large egg yolks
5 large egg whites
Salt and freshly ground pepper, to season
For the pastry: