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

In a large skillet, warm the olive oil over medium heat. Add in the chopped onions and shallots, then sprinkle with salt, pepper, and sugar.

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

Stir in the minced garlic and red pepper flakes, if using, and cook for about 1 minute until fragrant. Remove from heat.

In a medium bowl, combine your yogurt or sour cream, mayonnaise, grated cheese, and shredded mozzarella. Stir until smooth.

Fold the warm onion mixture into the creamy cheese base until well combined.

Spoon this mixture into the prepared baking dish and spread evenly.

Bake uncovered for 22–25 minutes, until hot, bubbly, and lightly golden. For an extra finish, broil briefly at the end.

Let it rest for about 5 minutes before serving, and enjoy with crackers, toasted bread, or fresh veggies.