In a medium sized mixing bowl, whisk together the 1 1/2 cup all purpose flour, 1/2 tsp sugar, 1/2 tsp salt, and 1/2 tsp baking powder.
Add the 6 Tbsp unsalted butter, melted and cooled to lukewarm and cold 3 Tbsp cold water and mix until the dough comes together into clumps. You don't want to see any dry flour left in the bowl, but try not to overmix.
Crumble the dough evenly over the bottom of your 10-inch tart pan with a removable bottom. Press the dough into the tart pan and up the sides of the pan, getting it as even as possible.
Chill the tart shell in the fridge for 20-30 minutes.
Meanwhile, preheat the oven to 400F.
to make the filling
In a small mixing bowl, whisk together the 1 egg plus 1 egg yolk, 1/2 cup heavy cream, 1/8 tsp salt, fresh cracked black pepper, 1 tsp Dijon mustard and 1/2 cup of the grated cheese (reserve about 1/4 cup for sprinkling on top of the tomatoes.)
Remove the tart shell from the fridge, and pour in the custard. Lay your tomato slices over the custard, and top with the remaining cheese.
Bake for about 35-45 minutes until the the custard is set and the crust is a light golden brown.
Remove from the oven and allow the tart to cool before carefully removing from the tart pan.
Slice and serve warm with fresh thyme leaves, if desired. Refrigerate leftovers.