At the base of an infinite number of recipes and preparations, from broth to mirepoix, onion adds its boost of flavour but very often disappears from our dishes, leaving room for the protagonist ingredients. In its pure version, a great classic of French cuisine, the onion is the key protagonist with its strong raw flavour, which is so sweet on our palate once it is cooked. 

Peel and slice the onions using a mandoline. 

Pour the oil in a 500 ml jar, add half of the sliced onions, the butter and the salt. 

Set the microwave (or the Ultracombi in microwave mode) to 500 W for 3 minutes and insert the jar to soften the onions. 

Remove the jar from the microwave and let it continue cooking outside the oven for 7 minutes. Season the remaining onion with the corn flour, the Worcestershire sauce, nutmeg, half of the grated Gruyère cheese and mix. Once 7 minutes have elapsed, open the first jar and mix the contents with a hand blender. Combine the raw onions, the cooked onion cream and 60 ml of hot water in a bowl. Divide the mixture into 2 x 500 ml jars, close and clamp, taking care to clean and remove grease from the edges. 

Set the microwave (or the Ultracombi in microwave mode) to 800 W for 5 minutes. Insert the jar and start the cooking cycle. 

Once the cooking cycle is completed, remove the jar and let it rest for at least 10 minutes. The onion soup will continue to cook. Shake the jar with rotating movements to mix everything. During this stage, the cooking cycle is completed with the vacuum being generated. 

The onion soup is almost ready; it is completed by simply adding a generous serving of grated Gruyère and use a blow torch to melt the cheese (alternatively, use the Ultracombi oven in grill mode). 

Store in the fridge for up to 8 days.