Butter an ovenproof dish. Cut the bread slices in small pieces. Transfer half of them into a bowl, and cover the bottom of the buttered pan with the other half.
Fill a saucepan with water and bring to a boil. Add the spinach and cook for one minute until just wilted. Drain with a strainer, pressing with a spoon to get rid of all the liquid.
Chop the spinach and add it to the bowl with the chopped bread.
Chop the onion. Heat ⅓ of the butter in a frying pan over medium heat. Add the onion and saute until translucent, about 5 minutes, stirring with a wooden spoon. Add to the bowl along with the cheese and eggs. Stir everything with a fork.
In the same frying pan you used for the onions, warm up the remaining butter with the half and half, until the butter melts. Do not let it boil.
Put the warm liquid into the bowl. Season with salt, pepper and nutmeg. Stir well with the fork.
Pour the mixture over the buttered pan lined up with bread pieces. Put the pan on a larger ovenproof dish or baking pan. Pour some water into the larger pan, until it reaches half way the height of the smaller pan.
Bake like this for 40 minutes. Take out of the oven, and let cool a little bit on a cooling rack before serving.
Recipe by PERU DELIGHTS at https://perudelights.com/meatlessmondays-get-strong-as-popeye-the-sailor-with-a-crustless-spinach-tart/