In a medium saucepan, melt the unsalted butter over medium heat. Once melted, add the finely chopped yellow onion and sauté for about 3-4 minutes, or until the onion is translucent. Add the minced garlic and cook for an additional 1 minute, stirring frequently to avoid burning.
Sprinkle in the all-purpose flour and stir constantly for about 1-2 minutes, allowing the flour to cook slightly and form a roux.
Gradually pour in the half & half, whisking continuously to smooth out any lumps. Continue cooking until the mixture thickens, about 3-5 minutes.
Once your mixture has thickened, add in the thawed and drained spinach and the chopped artichoke hearts. Stir until well combined.
Remove the saucepan from heat and stir in the shredded sharp Cheddar and grated Pecorino Romano until melted and fully incorporated.
Season the mixture with salt, freshly ground black pepper, and a pinch of nutmeg. Taste and adjust the seasoning if necessary.
If you prefer a baked dip, transfer the mixture to an oven-safe baking dish and bake in a preheated oven at 350°F (175°C) for 20-25 minutes, or until bubbly and golden on top. Serve warm with tortilla chips for dipping.
