Preheat your oven to 400°F (200°C).
Roll out the pie crust on a lightly floured surface to fit a 10-inch round baking sheet. Transfer the crust to the baking sheet and gently press it into place.
In a bowl, mix the mascarpone cheese, sumac, salt, and black pepper until well combined.
Spread the mascarpone mixture evenly over the crust, leaving a 1-2 inch border around the edges.
Arrange the sliced heirloom tomatoes on top of the mascarpone layer, overlapping them slightly.
Fold the edges of the crust over the tomatoes, pleating as necessary to create a rustic look.
Brush the crust with the beaten egg and drizzle olive oil over the tomatoes.
Bake in the preheated oven for 30-35 minutes, or until the crust is golden brown and the tomatoes are tender.
Remove from the oven and let cool for 5-10 minutes. Garnish with fresh basil before serving.