In a large sauté pan over high heat, add butter and heat until butter sizzles and has brown flecks (brown butter), lower heat to low. Add onion and sauté on low heat, stirring often until caramelized, about 40 minutes. Add garlic and sauté for another 10 minutes. Remove from heat.

In a medium sized bowl, combine sour cream, mayonnaise, fish sauce (if using), salt and pepper. Stir in onion mixture. Chill in the refrigerator for 1 hour or overnight, serve with cucumbers, dill pickles, olives, tomatoes, bell peppers.