In a large skillet over medium-high heat, melt the butter until bubbles.
Add the diced shallots and the sliced onions, and cook, stirring periodically, for about 20 minutes, or until golden brown.
At the end of cooking, add the minced garlic and sauté it gently until fragrant.
After allowing it cool for 12 to 15 minutes, transfer it to a cutting board.
Meanwhile in a large bowl, combine mayonnaise, sour cream, Worcestershire, celery salt, lemon juice, black pepper, and kosher salt.
Whisk until thoroughly combined.
Chop up the onion mixture once it has cooled, and add it to the bowl.
Mix everything together by folding it.
For at least two hours or overnight, cover the bowl and place it in the refrigerator.
Stir before serving, then top with chives before serving.