Preheat your oven to 375°F and lightly grease a small baking dish with a touch of olive oil.
In a small skillet over medium heat, sauté the diced yellow onion and minced garlic in the olive oil until soft and fragrant.
In a mixing bowl, whisk together the plain Greek yogurt, half of the shredded cheddar cheese, sea salt, black pepper, and dried thyme.
Thinly slice the Yukon gold potato into rounds approximately 1/8-inch thick using a mandoline or sharp knife.
Layer half of the potato slices in the bottom of the baking dish, followed by the shredded chicken breast and the sautéed onion mixture.
Spread half of the yogurt mixture over the chicken, then layer the remaining potato slices on top.
Spread the rest of the yogurt mixture over the top layer and sprinkle with the remaining cheddar cheese.
Cover with foil and bake for 20 minutes, then remove the foil and bake for an additional 10-15 minutes until the potatoes are tender and the top is golden brown.