Into a large skillet over low heat, add the oil and butter. When the butter is melted, add the onions.

Saute the onions for 35 minutes, stirring occasionally, until golden brown and caramelized.

Add the shallots and 1/4 teaspoon salt.

Continue cooking for 15 minutes, stirring occasionally, until onions and shallots are darker brown in color.

Transfer the cooked onions and shallots to a cutting board. Roughly chop into 1/4-inch pieces. Set aside to cool.

In a medium bowl, stir together the sour cream, mayonnaise, celery salt, steak sauce, remaining 1/2 teaspoon salt, and pepper.

Fold the onion mixture into the mayonnaise mixture.