Preheat oven to 350 F. In a large bowl, combine all ingredients, except the extra mozzarella, and mix well.: The dry, even heat of a preheated oven ensures the dip warms through consistently, and preheating prevents the casserole from sitting cold while the interior slowly comes up to temperature which can make the bake time unpredictable. You should feel a pleasant warmth in the kitchen as the oven reaches temperature, and the internal fan or indicator light will signal readiness. A common mistake is putting the dish into a cold oven, which can cause uneven bubbling and a less pronounced browned top. If your oven runs hot or cold, use an oven thermometer to confirm the real temperature and adjust as needed.
Scoop into a casserole dish, and bake, covered, for 40-50 minutes, until the mixture is hot and bubbly.: As you stir, notice the contrast between the creamy yogurt , the shredded cheeses, and the chopped artichokes . The aroma of garlic will lift as you mix, and the mixture should look uniformly pale with green flecks of spinach . Use a sturdy spoon to fully incorporate any clumps of cheese, because even distribution ensures even melting while baking. If the mixture seems too loose, it often means the spinach had excess moisture; squeeze it drier next time. Overmixing can break down the artichokes too much, so combine just until even.
Remove from the oven and turn on the broiler. Sprinkle a thin layer of mozzarella on top, and then place back in the oven, uncovered, for about 5-8 minutes, until the cheese is nicely browned. Remove from the oven and serve while still warm... with matzo, of course!: When transferring the mixture to the baking dish you should pack it gently so there are no large air pockets, and smooth the top with the back of a spoon. The surface should be even, not heaped, which helps the top brown uniformly. A ceramic or glass casserole helps retain heat and produces an even, bubbling center. A frequent oversight is using a dish that is too large, which can spread the dip too thin and cause it to dry out; choose a dish size that keeps thickness moderate.
Bake, covered, for 40 to 50 minutes, until the mixture is hot and bubbly: As it bakes you will hear faint bubbling and smell a rich, savory aroma that deepens with time. The edges will begin to bubble first and then the center, and small golden specks of cheese may appear at the perimeter. The heat allows the yogurt to meld with cheeses, creating a cohesive texture. A common problem is underbaking which leaves the center cool, so bring a spoon to the center to test for consistent heat. If the top browns too early, tent with foil while continuing to bake the interior.
Remove from the oven and turn on the broiler: Switching to the broiler concentrates intense, direct heat to the surface, which is why we reserve this step for finishing. The room will fill with that caramelized cheese aroma, and you will see the top go from pale to bubbling to flecked with golden brown spots. Keep the oven rack in the upper third but not touching the broiler element, and watch closely because browning happens quickly. A typical error is walking away and letting the top burn; set a short timer and stay nearby to catch the exact moment of perfect color.
Sprinkle a thin layer of mozzarella on top, and then place back in the oven, uncovered, for about 5 to 8 minutes, until the cheese is nicely browned: The fresh layer of mozzarella will melt into a glossy sheet and then bubble into small brown pockets, adding contrast to the creamy base. You want a light, even sprinkling so the top crisps attractively without forming a thick crust that hides the warm interior. When the cheese is bubbling and dotted with small amber spots, that is the cue to remove it. Avoid overbaking which makes the cheese rubbery; shorter, high heat is better for a tender, browned finish.
Remove from the oven and serve while still warm: The dip is at its best warm and slightly fluid, which makes it easy to scoop and gives the flavors full expression. As it cools, it firms slightly, but early bites while still warm will highlight the creamy mouthfeel and the contrast with the browned top. Serve with matzo or other approved dippers, and expect the first spoonfuls to draw the most praise. A typical mistake is letting it cool too long before serving which reduces the appeal, so time the broil finish to coincide with when guests are ready to eat.