Preheat your oven to 400°F and lightly grease a 9-inch tart pan with the olive oil.
Peel the sweet potato and slice it into very thin rounds using a mandoline or sharp knife.
Arrange the potato slices in the tart pan, overlapping them to cover the bottom and sides completely.
Bake the sweet potato crust for 15 minutes until the edges are slightly golden and set.
While the crust bakes, whisk together the egg whites, Greek yogurt, minced garlic, sea salt, black pepper, and dried oregano until smooth.
Place the fresh spinach in a pan over medium heat with a splash of water and cook until just wilted, then squeeze out all excess moisture.
Remove the crust from the oven and reduce the heat to 375°F.
Spread the wilted spinach over the bottom of the crust, then pour the egg white mixture over the top.
Arrange the heirloom tomato slices on top of the egg mixture and sprinkle with crumbled feta and parmesan cheese.
Bake for 25 to 30 minutes until the center is firm and the cheese is beautifully melted.