Preheat the oven to broil.
Heat olive oil in an oven proof skillet over medium-high heat; add pancetta. Cook 2 minutes; add onion and cook 2 minutes. Stir in potato, and cook 10 minutes or until tender. Stir in spinach leaves; cook 1 minute until wilted. Season with salt and pepper.
Meanwhile, whisk together eggs, egg whites, salt, and pepper. Pour over the spinach mixture and cook, just until the eggs begin to set. Sprinkle cheese evenly over the top, then place pan under the broiler for 3 minutes, until fritatta rises slightly, becomes light and settled, and cheese bubbles.
Remove from the oven, and sprinkle the top with thyme leaves. Transfer to a serving plate, cut into 6 pie-shaped wedges and serve.