Peel and slice the onions into half-moon shapes. In a pot, melt the butter and add the oil – avoid colouring or burning the butter. Add the onions and cook gently until soft but not browned, then add the salt. If the onions begin to catch, add a touch of water and continue cooking.
Drain the onions and squeeze as much liquid from them as possible. Spread onto a tray and place in a smoker and hot smoke at 70–80°C for 25 minutes.
Remove the onions from the smoker, place in a Thermomix jug and blend on high until a smooth purée forms. Remove from the blender and cool over an ice bath. Reserve in a vacuum-sealed bag.
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