Heat 1 tablespoon of the extra virgin olive oil in a medium-sized frying pan over medium heat. Add the onion and stir to coat in the oil. Reduce heat to low and continue cooking until the onions have softened and turn light gold in color, about 35 to 45 minutes, stirring every 5 minutes. Remove from heat, stir in the balsamic, and set aside to cool slightly.

In a medium-sized bowl, mix together the ricotta, egg, salt, garlic powder, and caramelized onion mixture. Set aside.

Preheat the oven to 400 degrees Fahrenheit.

For the crust, mix together the flour and salt in a medium-sized bowl. Cut the ghee into pea-sized pieces over the bowl, stirring every few minute to incorporate and coat the ghee pieces with the flour. Add the ice water in 4 increments, stirring after each addition. Pinch the dough to mix until it holds together when you squeeze it in your hand. Cover the dough and place it in the freezer for 15 minutes. Roll out the dough and transfer it to a 9-inch tart pan, pressing it into the bottom and sides of the pan. Trim the edges and discard the excess.

Pour the ricotta mixture into the pan, leaving about 1/4 inch of space between the top of the crust and the top of the ricotta mixture. Add the heirloom tomato slices to cover the top of the filling, then brush the exposed slices with the remaining 1 teaspoon of olive oil. Sprinkle with the zaatar seasoning, place the tart pan on a baking sheet, and place the baking sheet in the oven (that way any melted ghee or tomato juice that might drip out of the pan won't start burning the bottom of your oven). Bake until the crust is completely cooked, and the tomato slices on top have wrinkled slightly, about 45 to 50 minutes.

Remove from the oven and allow to cool for 30 minutes. Garnish with the fresh basil and a pinch of flake sea salt and serve.