Preheat oven to 350 degrees F and gently oil a baking dish.
If you haven't already, dethaw frozen spinach by placing it in a fine strainer and running hot water over it. Let sit and drain of water. Before adding to dip, gently squeeze out excess water.
Meanwhile, in a medium saucepan, heat 1-2 tablespoon olive oil over medium heat until shimmering, not burning, then add onions and gently stir. Sprinkle with salt. After they start to sizzle, reduce heat to low and let cook, stirring frequently until golden and caramelized.
When onions are almost done, in a mixing bowl, combine cream cheese, mayo, pecorino, parsley, lemon juice and scallions. Mix until well combined and smooth.
Add caramelized onions and spinach and mix until well combined.
Transfer to the prepared baking dish.
Bake for 15-20 minutes until warm.
Give a stir if desired and serve with your favorite gluten-free crackers, chips or carrot and celery sticks. Enjoy!