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1
Combine the sugar and water in a heavy saucepan. Bring to the boil over medium heat, stirring only until the sugar dissolves. Continue without stirring until the syrup reaches 120 °C on a thermometer — the ball stage. At this temperature the syrup will form a soft, pliable ball when a drop is pressed between wet fingers.
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2
Add the prepared fruits to the syrup at 120 °C. Stir once to submerge. Reduce the heat to a gentle simmer and cook until the fruit is tender but visibly intact — firm fruits (pears, orange rind) should yield to the tip of a knife without collapsing; soft fruits (cherries, figs) should hold their form. Remove from the heat.
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3
Allow the syrup and fruit to cool, undisturbed, until the temperature falls below 35 °C. This is where the dish lives or dies: the mustard essential oil is a volatile compound that evaporates above 40 °C — add it too soon and the defining heat of the mostarda is lost entirely. Confirm the temperature with a thermometer before proceeding.
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4
Stir the essenza di senape into the cooled syrup. The oil will disperse through the liquid. The aroma should be immediately sharp and eye-watering at close range — a vaporous, wasabi-like assault that vanishes within seconds. If no such aroma is detectable, the oil has been added while the syrup was still too warm.
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5
Transfer the fruit and syrup into a sterilised 1-litre jar, ensuring the fruit is fully submerged. Seal and store at room temperature. The mustard oil infuses the fruit gradually over time. Do not taste for balance until at least 2 weeks have elapsed; 4 weeks produces a fully integrated result. At the 2-week mark, a piece of fruit should carry the syrup's sweetness on the palate followed by the brief, vaporous mustard heat — both clearly present, neither overwhelming the other.