Heat a large sauce pan over medium heat and add in the butter or oil. When it's hot, add in the onions and let cook, stirring occasionally, for about 5 minutes or until the onions begin to soften. Stir in the corn (frozen or thawed) and garlic, and let cook until the corn is thawed and soft.
Stir in the cream and raise the heat to medium-high. Stir together the cornstarch and water in a small bowl, making sure no lumps remain. When the corn mixture comes to a heavy simmer, stir in the cornstarch mixture. When the corn starts to thicken, lower the heat to medium-low and stir in the barbecue sauce. Let it simmer for 2-3 minutes, then remove from heat and season to taste with salt and pepper. Serve warm garnished with chopped parsley or cilantro. Note: Can be made ahead and reheated.