Position a rack in the center of the oven and preheat the oven to 350F. Lightly oil a 4- to 6-cup baking dish.

Heat the oil in a small skillet over medium heat. Add the onion and cook, stirring often, until softened, about 3 minutes. Stir in the garlic and cook until fragrant, about 1 minute more. Transfer to a medium bowl and cool slightly.

Add the cream cheese and mayonnaise to the bowl. Using a silicone or rubber spatula, mash them together to combine. Add the 1/2 cup of the Parmesan, the Romano, and hot pepper sauce and mix well.

A handful at a time, squeeze the excess liquid from the spinach, and add the spinach to the cream cheese mixture. Add the artichoke hearts and mix well. Spread in the baking dish and sprinkle with the remaining Parmesan. (The dip can be covered with plastic wrap and refrigerated for up to 1 day.)

Bake until the dip is bubbling and browned, about 30 minutes. Serve hot, with the baguette slices for dipping.