Peel the onions' outermost layers, leave the inner ones. Wash with cold water and place each head in a sheet of aluminum foil.
Cut each head into 4, without slicing down all the way through. Then make several more cuts.
Pour 1 tbsp olive oil over each, sprinkle salt, black pepper, powdered or sliced garlic and a pinch of oregano.
Put a few leaves of rosemary in each head as well.
Fold the aluminum foil into a packet and place the onions in a preheated 356°F (180 °C) oven to bake for 30 min.
Serve as a garnish to a main course consisting of meat or fish.