Preheat your oven to 350° F. Line a large baking sheet with parchment paper.
Slice your tomatoes into thick, round slices. Using a small paring knife, remove the internal parts of the tomato from each slice (MAKE SURE YOU LEAVE LEAVE A NICE THICK BORDER ON THE TOMATO SLICES). Chop up the insides into small pieces. Set your round pieces on your baking sheet.
In a medium bowl, beat your eggs. Next add in the chopped tomato pieces, chopped spinach, feta and seasonings and mix to combine. Fill each tomato round with the mixture. Top each with some shredded mozzarella and bake for 25 minutes or until tops are golden brown.