Preheat oven to 400 degrees.
Arrange the tomato halves on a cooking sheet and roast in the oven for 13 minutes. Let them cool a bit.
Blend the egg yold, lemon juice and lemon rind in the food processor until smooth. Slowly pour in the EVOO while still running. Mix in the tuna and capers to the mix. Season with salt and pepper.
Spoon this tuna mix evenly into the tomato halves and cover with mozzarella cheese. Put in the oven for 3-5 minutes.