- Preheat the oven to 180°C/gas 5. Remove the papery outer layer of the onions, leaving their skins intact, then trim off their tops and bottoms.
- Cut a cross in the top of each onion, slicing down to cut into four, but only three quarters of the way through, making sure the onions stay whole.
- Place each onion on a square of foil and create a loose parcel that's open at the top. Cut 4 cloves garlic into 5 slices each, then push one slice into each of the cross cuts and one in the centre, pushing down to the core.
- Do the same with the rosemary spigs, using one for each onion, breakingthe sprigs into smaller pieces.
- Add balsamic vinegar, olive oil and a grind of pepper to each parcel. Then, bring up the corners of the foil and twist it at the top to seal. Bake for 45 minutes.
- Open up the foil parcel at the table to enjoy the aromas that are sealed in.
- Use smaller onions, but reduce the cooking time by 10 minutes.
- Use fresh sprigs of thyme or sage as an alternative to rosemary.