Preheat your oven to 375°F (190°C).

In a large bowl, combine the flour and salt. Add the cubed butter and mix with a pastry cutter or your fingers until the mixture resembles coarse crumbs.

Gradually add the ice water, one tablespoon at a time, mixing until the dough comes together. Form the dough into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

On a lightly floured surface, roll out the chilled dough into a 12-inch circle. Transfer it to a 9-inch tart pan, pressing it into the bottom and up the sides. Trim any excess dough.

In a medium bowl, mix together the ricotta cheese, Parmesan cheese, egg, oregano, garlic powder, salt, and pepper until smooth. Spread the mixture evenly over the tart crust.

Arrange the sliced heirloom tomatoes on top of the ricotta mixture. Drizzle with olive oil and season with a pinch of salt and pepper.

Bake in the preheated oven for 35-40 minutes, or until the tart is golden brown and the tomatoes are tender.

Remove from the oven and let cool for 10 minutes. Garnish with fresh basil leaves before slicing and serving.