1. Preheat oven to 375 degrees Fahrenheit (190 degrees Celsius).
2. Cover the bottom of a 9×13-inch (23×33-centimeter) baking pan with puff pastry, using your hands to spread it out a little. Bake for 10 minutes.
3. Meanwhile, heat olive oil in a large frying pan. Add onion slices and season with some salt. Sauté over medium-low heat for around 20 minutes, making sure to mix every now and then to prevent burning. The onions should be golden. Add the vegetable stock, silan, soy sauce, 1/4 teaspoon salt, and pepper. Allow to simmer for another three to five minutes, or until reduced and glazing the onions.
4. Cover the par-baked puff pastry with a layer of mozzarella slices. Arrange the glazed onions on top. Cover the onion mixture with another layer of mozzarella. Place a thin layer of puff pastry on top of that. Smear with beaten egg, then sprinkle with salt and granulated garlic.
5. Bake for 25–30 minutes or until the puff pastry is golden. Once slightly cooled, cut into squares or triangles and enjoy!