Preheat oven to 400 F

Add butter to a 12 inch cast iron skillet ( or you can use an enameled Dutch oven -le Creuset, staub-etc.) Melt butter over medium heat till foamy.

Add the leeks and cook till softened. Increase the heat and add mushrooms. Season with salt and pepper.

When veg browned add spinach till it's wilted, this will take less than a minute. Transfer mixture to a large bowl and add cubed bread, and cheese. Mix.

In another bowl prepare the custard part. Add eggs, half and half, thyme, and rosemary... whisk to combine. Pour the mixture over the bread and let sit for at least 30 minutes-Stir occasionally, without breaking the mixture. You can also do this the night before and cook the next day.

Core and slice the tomatoes. 

Spoon the bread mixture into the cast iron and lay the tomatoes nicely in any design you would like. Add the parm cheese half way through the baking. It should take about 25-30 mins baking. Remove from oven and let it sit for atleast 5 mins. Unmold and place on a nice serving platter or serve as is straight from the skillet!!!