In a large skillet, melt 2 tablespoons of butter over medium-low heat. Add the thinly sliced yellow onions and cook them slowly for about 30-40 minutes, stirring occasionally, until they are deeply caramelized and jammy.

While the onions are caramelizing, take 1 cup of small-curd cottage cheese and place it in a fine-mesh sieve over a bowl. Allow it to drain for at least 20 minutes, pressing gently to remove excess liquid.

Preheat your oven to 375°F (190°C). Grease a 9-inch pie dish or baking dish with butter or cooking spray.

In a blender or food processor, combine the drained cottage cheese, 3 large eggs, and 2 tablespoons of cornstarch. Blend until the mixture is smooth and frothy.

Combine the creamy cottage cheese mixture with 2 teaspoons of fresh thyme, sprinkle with salt and black pepper to taste, and gently fold in the cooled caramelized onions and ¾ cup of grated Gruyère cheese.

Carefully pour the combined mixture into your prepared dish, spreading it evenly. Sprinkle the top with the remaining ¼ cup of Gruyère cheese.

Place the dish in the preheated oven and bake for 35-40 minutes. The edges should be set, and the top should be golden brown.

Let the dish cool for 10-15 minutes before slicing into wedges.