In a large skillet over medium heat, cook bacon until crisp. Remove and drain on paper towel. Crumble. Remove all but 2 Tbsp. bacon grease from skillet.
Add butter to skillet with bacon grease. Stir in sweet and red onions and cook over medium heat until soft (about 5 minutes), stirring frequently. Stir in 1 Tbsp. corn syrup. Continue to cook for 10 to 15 minutes until onions are caramelized.
Stir in remaining 1 Tbsp. corn syrup, vinegar and crumbled bacon. Mix well and remove from heat. Add sour cream and cream cheese, stirring to blend. Stir in 1-1/2 cups Monterrey Jack cheese; mix well. Pour into greased 1-qt. baking dish. Top with remaining cheese.
Bake in a preheated 350-degree oven for 15 to 20 minutes or until cheese is bubbly around the edges. Sprinkle with green onions and serve.