Heat the oil in a skillet over medium heat. Add the bacon and cook until crispy. Transfer to a plate lined with paper towels to drain excess oil, leaving the rendered fat in the pan.

In the same pan, add the sliced shallots and onions. Sauté on medium-low heat for about 20 minutes, stirring occasionally, until golden and caramelized. In the last 5 minutes, stir in the balsamic glaze and brown sugar to enhance the sweetness.

In a large bowl, combine the cream cheese, sour cream, cheddar, mozzarella, crispy bacon (reserve a bit for topping), and caramelized onions (save some for garnish). Mix until smooth and season with salt to taste.

Transfer the mixture to a baking dish, smoothing the top. Sprinkle with the reserved cheese, bacon, and onions. Bake at 350°F (175°C) for 10 minutes, then broil for 2-3 minutes until bubbly and golden.

Serve warm with baked crostini or crunchy tortilla chips. Garnish with parsley seasoning if desired for a pop of color.