Heat the olive oil in a medium-sized nonstick skillet over medium-high heat. Add the onions and sauté, stirring occasionally, until the onions begin to sweat, about 5 minutes.

Decrease the heat to medium and continue cooking, stirring occasionally, until the onions begin to turn deep brown, about 25 minutes.

Add a couple of tablespoons of water to deglaze the pan and decrease the heat again to medium-low. Continue cooking until onions are deep brown in color and caramelized, about 20 more minutes. Set aside until ready to use.

Preheat the oven to 425° F. Add the spinach to a pot and cover it. Heat over medium, stirring occasionally until wilted, about 3-5 minutes.

Once the spinach is cooked, add the remaining ingredients (except for ½ cup of the mozzarella cheese) to the saucepan. Stir and heat until it's all well-combined, about 5-8 minutes.

Transfer the hot dip to a small oven-safe casserole dish and sprinkle it with the remaining cheese.

Bake for 20-25 minutes in the preheated oven, or until the cheese is golden-brown and the dip is bubbly. Serve with potato chips, and enjoy.