First, make the dough for the crust. Whisk the flour, salt, and dried tarragon together in a bowl to aerate them. Then use a pastry cutter to cut in the butter until it becomes a pebbly mixture. Finally, stir in the milk until the dough comes together. If it is still too dry, pour in a little more milk until it comes together. Bring it together into a firm ball with your clean hands in the bowl, then cover the bowl and let it chill for an hour in the refrigerator.

While the dough chills, prepare the filling. Heat the oil in a large skillet over medium high heat. Add in the onion, garlic and basil and let them soften for a minute to become really fragrant. Pour in the lemon zest, lemon juice and white wine and let them cook off for another 30 seconds. Finally, add in the chopped shrimp and scallops to cook for another 30 seconds. While they cook, season the mixture with the salt, pepper and tarragon. When it's done, set it aside. In another bowl, whisk the egg until it is foamy. Then keep whisking while you pour in the heavy cream until it is a gorgeous custard.

Now it's time to assemble! Preheat the oven to 400 and line a sheet tray with a silicone mat. Take out the ball of dough and press it out into a disc. Roll it out into a circle about 10 inches in diameter, then transfer it to the sheet tray. Spread the spinach out in a bottom layer, leaving a large border. Then pour the shellfish mixture into a layer on top of the spinach bed. Pour the custard over everything. If a little of it leaks out onto the tray, it's ok.

Bring the border of dough over to enclose the filling a little, then sprinkle the parmesan on top. Bake the galette for 30 minutes, until it becomes bubbly with a golden crust. Let it cool for a couple of minutes, then cut it into quarters. Serve immediately with more chopped basil on top for garnish and enjoy!