Melt 2 tablespoons butter in a saute pan. Add olive oil. Stir and heat over medium-high heat. Add onions, shallots and a pinch of salt.
Cook without stirring until onions start to soften and color, about 2 or 3 minutes.
Caramelize onions by stirring them and reduce heat to medium. Add 2 tablespoons butter and continue cooking onions until golden brown, stirring occasionally to prevent sticking or burning.
Once onions have reached their desired color, add champagne or white wine. Scrape bottom of pan to pick up any caramelized bits and fold them back into onion mixture. Remove from heat and stir in thyme and garlic powder.
Place the sour cream, mayonnaise, one half of scallions, salt, pepper, lemon zest and capers in a mixing bowl. Add caramelized onion mixture to the sour cream mixture and stir to combine all ingredients.
Garnish the dip with chopped scallions and chives. Serve with potato chips or crudites.