Halve the loaf lengthwise almost all the way through, leaving it attached on one side.
Open it, lay it flat and brush the insides with 2 to 3 tablespoons olive oil.
Tuck in the mozzarella, then sprinkle with salt and ¼ teaspoon oregano.
Close, then wrap tightly with foil.
Place on a sheet pan and bake for 20 minutes.
Meanwhile, in a medium saucepan, over medium-high heat, heat the remaining 2 tablespoons of oil, garlic, remaining ½ teaspoon oregano and red-pepper flakes, stirring occasionally, until fragrant; bout 1 minute.
Pour in the marinara sauce and adjust the heat to bring it to a gentle boil.
Stir in the sherry vinegar, if using.
Reduce the heat to maintain a gentle simmer, stirring occasionally, until the bread is ready; about 15 minutes more.
Season to taste with salt and pepper.
Transfer the sauce to a bowl or serve from the pan; top with grated Parmesan.
Slice the bread, or let everyone pull the pieces apart themselves.
Keep wrapped until ready to serve.
Notes
Try brushing the bread with garlic and oil, or melted butter, before adding the mozzarella — makes a delicious cheesy jazzed up garlic bread.