Heat a large skillet over medium heat and add olive oil. Once hot, add thinly sliced onions along with salt and pepper. Cook for about 15 minutes, stirring frequently until they turn golden brown.

Lay out your thinly sliced flank steak on a cutting board. Season both sides with garlic powder, dried thyme, salt, and black pepper. Ensure the steak is thin enough to roll easily.

Spread an even layer of caramelized onions over the steak. Sprinkle with Gruyère cheese and parsley, then roll the steak tightly from one end to the other. Secure with toothpicks or kitchen twine.

Preheat your grill or grill pan over medium-high heat for about 5 minutes. Cook the pinwheels for 4-5 minutes per side, turning until the steak is cooked to your desired doneness.

Remove pinwheels from the grill and let them rest for about 5 minutes. Remove toothpicks or twine, slice into bite-sized pieces, and serve warm.