Make a "Pâte brisée" (or you can also buy an unsalted or savory pie crust in your grocery shop, but it's not the same).
Quickly combine flour and butter in a bowl using only your fingertips and being careful not to over mix otherwise the butter melts (it's fine if remain a few butter pieces big like peas) until you obtain a kind of a crumble. Create a hill with the crumble and with 1 finger make a hole on top of it, add the salt and pour the water. Mould first with a single finger and then with the whole hand to create a ball being careful not to over mix. Wrap the dough in a plastic foil and refrigerate for at least 30 min (you can also keep the dough for 1 day and make the tart the next day).
In the meanwhile make the filling: Slice the leeks very thinly. Overcook them in a covered pan with the water and the salt for 30 minutes over medium-low heat. Sprinkle with the flour and keep stirring over medium-low heat to create a mixture similar to besciamelle, if it is necessary add a little milk to adjust the thickness. When the mixture is dense (about 10 min) adjust salt and add the diced bacon and optional grated parmigiano cheese. Set it aside.
Preheat the oven at 375-400 F.
Dust a wooden surface with flour and roll the dough into an about 1/8" thick round. Grease a 9-10" round baking tin using oil or butter, dust it with a little flour and put the dough in. With the tines of a fork pierce several times the dough for it not to bubble while baking. Pour the leeks mixture over the crust. Fold the borders of the crust over the mixture making pleats as you fold it, if needed. Bake the tart at 375-400 F for about 30 minutes or until the crust is golden brown.
Serve warm with a mixed greens or fennel salad seasoned with extra virgin olive oil, fresh erbs, and a little salt or let the tart cool and serve the next day.