Melt butter in a large skillet over medium-low heat and cook sliced onions for 30-40 minutes until caramelized.

Drain cottage cheese in a fine-mesh strainer for at least 20 minutes to remove excess whey.

Preheat oven to 375°F (190°C) and prepare a 9-inch pie dish by greasing it lightly with butter.

Blend drained cottage cheese, eggs, and cornstarch in a blender until smooth.

In a mixing bowl, combine blended custard, salt, pepper, thyme, caramelized onions, and Gruyère cheese.

Pour mixture into greased pie dish, sprinkle remaining Gruyère on top, and bake for 35-40 minutes.

Allow to cool on a wire rack for 10-15 minutes before slicing and serving.