Heat a pan. Add the butter and saute the sliced onions and garlic with salt and pepper for 12-15 minutes over a medium flame until it is caramelized. Remove from the flame and cool.
To a bowl, add the cream cheese, Greek yogurt, cilantro, green onions, caramelized onion mixture, cayenne, serrano pepper, cheddar and oaxaca cheese. Mix thoroughly.
Preheat the oven to 350℉.
Transfer the onion-cheese mixture to two 4-in baking bowls or to smaller ramekins. Sprinkle with Cotija cheese. Bake for 15-17 minutes until the cheese misxture bubble and melted. Remove from the oven.
Serve warm with chips, peppers, tortilla, naan, or pita.