Heat 2 tablespoons of olive oil in a large skillet over medium heat. Add 1 sliced onion, seasoning with salt and pepper. Sauté for about 15 minutes until deeply caramelized and golden brown.

Once the onions are caramelized, mix in 1 teaspoon of garlic powder and 1 teaspoon of dried thyme, followed by a splash of balsamic vinegar. Cook for an additional minute and let the mixture cool.

Lay each thinly sliced flank steak on a cutting board. Spread cooled caramelized onion mixture over each slice, then sprinkle with Gruyère cheese and fresh parsley.

Tightly roll each steak slice into a pinwheel and secure the ends with toothpicks or kitchen twine.

Preheat your grill or grill pan over medium-high heat. Grill the secured pinwheels for about 4-5 minutes on each side until golden-brown.

Transfer the pinwheels to a cutting board and let them rest for 5 minutes. Remove toothpicks or twine, then slice into bite-sized pieces and serve.