Preheat your grill to a medium-high heat, around 400°F (200°C).

In a mixing bowl, combine olive oil, unsalted butter, grated garlic, kosher salt, black pepper, and red pepper flakes.

Lay out a piece of heavy-duty aluminum foil and layer the potato slices, yellow onion, and fresh thyme.

Drizzle the olive oil mixture over the layered ingredients in the foil.

Seal the edges of the foil tightly to keep moisture inside.

Place the foil packets on the grill and cook for about 25–30 minutes, flipping halfway.

Remove the packets from the grill, let them rest for a few minutes, then serve hot.