In a large skillet, saute the leeks in olive oil until softened and lightly browned. Set aside to cool.
MAKE THE CRUST:
Preheat the oven to 350℉. In a medium bowl, combine the flour, cornmeal, sugar and salt- whisk together. Add the butter and use a pastry cutter to cut it into the dry mixture (you can also do this step in a food processor). Keep cutting in the butter until your mixture is a mess of crumbs. Add 1/3 cup of buttermilk and stir to combine. You'll need to add 1 to 2 more tablespoons of buttermilk to create a nice ball of dough (get in there and use your clean hands... and transfer it to a floured surface where you can knead it into a ball). You don't want it to be wet, so don't add too much buttermilk- just until it all comes together. If you're doing this in a food processor, just drizzle in the buttermilk until the dough comes together in a ball. Place the ball of dough onto a large parchment-lined baking sheet and roll it into a 12 to 14-inch round circle (it does not have to be perfect).
ASSEMBLE AND BAKE THE TART:
Sprinkle the leeks around the middle of the rolled out crust, leaving about a 1½-inch border around the edge. Add cheese next, then tomato and zucchini. Gently fold edges over toward the middle, on top of the filling. Sprinkle fresh herbs on top and a good dose of salt and pepper.
Bake for 45 minutes to 1 hour, or until the crust is golden brown and the filling is bubbly. Let cool for about 15 minutes, then slide the tart onto a cutting board, cut into slices and serve.