Preheat oven to 200°C.
Place whole onions into a baking dish, complete with skins on. Drizzle with enough olive oil to just coat.
Turn down oven temperature to 180°C and roast onions for 1 hour.
Remove from oven, cool and peel.
Place the flesh of the onions, as much of the residual juices from the pan as possible and the 100g olive oil into the Thermomix bowl. Blend 1 min/speed 7.
Add salt and lemon juice and blend 1 min/speed 8.
If not a pourable consistency, add more olive oil until you are happy with the texture.
Store in airtight container in the fridge. It will pour more easily if warmed before use.