This recipe is a tradition that snuck up on my family. My grandmother Debbie made it every Passover and Rosh Hashanah for as long as I can remember and, my mom says, for as long as she can remember, too. It remains a constant on our table to this day, long after Grandma Debbie’s passing, and I couldn’t imagine our holiday dinner without the crisp-topped mashed potato casserole with its ribbonlike layer of spinach, generously scooped on the “good china” and ready to be pooled with brisket gravy.