Preheat the oven to 350 degrees.
Add the oil in a skillet over medium-high heat.
When the oil is hot, add the diced onion and sauté until fragrant, about two (2) minutes.
Add the chopped Brussels sprouts and spinach. Add a bit of sea salt and freshly cracked black peppercorns, and sauté for four (4) minutes, then remove from heat. Set aside.
Add the softened low-fat cream cheese, roasted garlic, sour cream, shredded mozzarella, parmesan cheese, and mayo in a separate bowl. Stir to combine.
Add the warm Brussels sprouts mixture to the cream cheese mixture and stir gently to combine.
Brush an oven-safe dish with oil or a melted butter and oil mixture—about a tablespoon will do.
Spoon the mixture into the oiled dish and bake on the center rack for 15-20 minutes until bubbly and slightly golden brown on top. [optional: add green onion as a garnish]
Serve with tortilla chips and wedges of fresh veggies like peppers, celery, and onion.