Start by heating your oven to 300°F and coating a 9-inch tart pan with cooking spray. Then place 1 ¾ c. of the pistachios on a baking sheet and bake for 8 minutes, or until lightly toasted. Then remove from the oven and allow to cool for at least 15 minutes. Increase the oven temp. to 350°F.
Next, chop the toasted pistachios in a food processor until finely ground, about a minute. Mix the ground pistachios and fine sea salt together and set aside.
Then beat the egg whites on high with an electric mixer until stiff peaks begin to form. Gently fold the egg whites into the ground pistachios before pressing the mixture into the bottom and sides of the tart pan. Placing the tart pan on a baking sheet and bake for 20 minutes or until browned. Remove from the oven and allow to cool for at least 30 minutes.
When the crust is cooled, spread the apricot preserves evenly across it. Then bring the half-and-half to a soft boil in a saucepan before pouring it over the chopped chocolate. Allow to sit for a minute before beginning to stir. Stir until the mixture is smooth, then pour over the top of the apricot preserves. Place the tart into the fridge and chill for at least 2 hours. Last, chop the remaining pistachios and sprinkle over the top of the tart with flaky sea salt.