Preheat the oven to 350°F and set an oven rack in the upper third of the oven.
Heat oil in a medium skillet over medium heat. Add onions and garlic, and cook until softened, 4 minutes. Season with salt and pepper. Add spinach and cook until wilted, 2 minutes.
Transfer the cooked spinach mixture to an 8x3 oven-safe baking dish. Add the artichokes, Greek yogurt, tahini and 1 cup mozzarella cheese,and stir to combine. Sprinkle the top with feta and remaining ¼ cup mozzarella cheese. Wipe any messy edges with a paper towel.
Bake for 20-22 minutes, or until heated through and the cheese on top is melted. Switch oven to a high broil and place dip on top rack. Broil until cheese is golden brown and bubbling, 2 minutes. Serve warm with pita chips.