Preheat your oven to 375°F and lightly grease a small to medium baking dish. Set it aside.

In a large skillet, warm the olive oil over medium heat. Add the finely chopped onions and sliced shallots, followed by the salt, pepper, and sugar.

Cook slowly for about 20 to 25 minutes, stirring occasionally, until the onions are golden, sweet, and soft.

Once the onions are beautifully caramelized, stir in the minced garlic and crushed red pepper flakes (if using). Cook for about 1 minute until fragrant, then remove from heat.

In a medium bowl, whisk together the Greek yogurt or sour cream, mayonnaise, grated cheese, and shredded mozzarella until smooth and well combined.

Gently fold in the warm onion mixture, ensuring everything is evenly incorporated.

Spoon the mixture into the prepared baking dish, spreading it out evenly.

Bake uncovered for 22 to 25 minutes, until hot, bubbly, and slightly golden on top. Broil for a minute if you prefer extra color, but watch carefully to avoid burning.

Let the dip rest for about 5 minutes before serving. Enjoy warm with crackers, toasted bread, or fresh-cut veggies.