By hand or by pulsing in food processor cut butter and shortening into the flour until butter and shortening are pea sized. Add salt and ice water one tbsp at a time pulsing or mixing until dough just starts to come together. Be careful not to over mix. Using your hands form a ball with the dough, wrap up in saran wrap and chill for 30 minutes.
On a lightly floured surface roll out dough in a circle until ¼” thick. Carefully press the dough into a tart pan, cutting away excess dough. Puncture dough with a fork a couple of times and chill for 30 more minutes.
Preheat oven to 400°F. Fit a sheet of foil into the inside of the pie crust until crust is completely covered, using either pie weights or dry rice, fill the foiled area. Bake for 20 minutes. Remove foil and weights and continue to bake for another 10 minutes, or until crust is just set.
While you are baking your crust, melt 4 tbsp of butter and 2 tbsp oil in large sauce pan. Reduce heat to low and add onions, leeks, scallions and shallots. Cover and cook for 10 minutes. Remove cover and continue to cook, stirring often, until onions turn a golden brown color. This process should take about 30 minutes. Remove from heat and allow to cool to room temperature.
In a large bowl whisk together eggs, creme fraiche, salt, nutmeg, pepper and chives. Add onion mixture and stir until onions are evenly distributed throughout the mixture. Pour egg mixture into the pie crust and continue to bake for another 25-35 minutes or until eggs are set and the crust is golden brown.
Just before serving toss arugula with olive oil, salt and pepper. Add the salad to the top of the tart and serve.
Tart can be made up to a day ahead. To warm up, cover and bake at 300°F until warmed through.