Preheat oven to 400°F and line a baking sheet with parchment paper.
Peel and cube the sweet potato into 1/2-inch pieces, then toss with 2 teaspoons of the olive oil, salt, pepper, and dried oregano.
Spread the sweet potatoes on the baking sheet and roast for 20-25 minutes until the edges are golden and caramelized.
While the potatoes roast, heat the remaining teaspoon of olive oil in a non-stick skillet over medium heat.
Add the spinach to the skillet and sauté until just wilted, then remove and set aside.
Wipe the skillet if needed, then pour in the egg whites and cook undisturbed until the bottom is set and the top is almost firm.
Place the sautéed spinach and crumbled feta on one half of the egg whites.
Carefully fold the omelette in half and cook for another minute to allow the cheese to soften.
Slide the omelette onto a plate and serve immediately alongside the roasted sweet potatoes.