In a large bowl, stir together the chopped spinach, mayonnaise, Parmesan cheese, shallot, garlic, salt, pepper and cayenne until completely combined. Gently fold in the Greek yogurt until thoroughly incorporated. Cover and refrigerate for at least 1 hour.
Serve in a bread bowl with reserved bread cubes, or simply serve in a bowl with fresh vegetables, crackers, tortilla chips or pretzels. Leftover dip can be stored in an airtight container in the refrigerator for up to 3 days.
Fresh Spinach- To make this with fresh spinach, use 1 lb of fresh spinach. Cook down until wilted, about 3-4 minutes. Drain completely before using in the dip.
Make Ahead- You can absolutely prepare this ahead of time; you can mix it up, cover and refrigerate up to one day in advance. It's good for about three days in the refrigerator, but I recommend only making it a day in advance so it's at peak freshness!
Storing- Keep the leftover dip in an airtight container for up to 3 days. I would suggest storing the dip separate from the bread and your best bet is to eat the bread the same day you serve the dip.
Freezing- I don't recommend freezing it, however, because the yogurt and mayo can break down a bit when thawed and cause the dip to be runny.