Preheat your oven to 375°F (190°C). Thinly slice the sweet potatoes to about 1/8-inch thickness.

Grease a 9-inch pie dish lightly with olive oil. Arrange the sweet potato slices in a spiral pattern.

Spray the assembled crust lightly with olive oil and season with salt and pepper. Bake for 15 minutes.

In a skillet, heat olive oil over medium heat. Sauté diced onion for 5 minutes, then add garlic and spinach until wilted.

Whisk together eggs and Greek yogurt until fluffy. Add feta cheese, salt, and pepper.

Remove the crust from the oven. Layer the sautéed spinach mixture, then pour the egg mixture over, and top with sliced tomatoes.

Bake for 30-35 minutes until the egg is set and the top is golden brown.

Let the quiche cool for 5-10 minutes before slicing. Serve warm or allow to cool completely.