Start by making the pickled onions. Using a sharp knife or mandolin, slice the onion into very thin half-moons. Place in a bowl with the sugar and red wine vinegar, toss to separate the slices and coat well, then set aside at room temperature to pickle, stirring occasionally while you make the rest of the meal.
Cook your grain of choice according to the packet instructions.
To make the lemon vinaigrette, combine the lemon juice, apple cider vinegar, honey, salt and pepper in a bowl or jar. Gradually whisk in the olive oil until emulsified. Set aside.
Heat the oil in a small frying pan over medium-high heat. Slice the halloumi into 1cm-thick pieces and arrange in the pan in a single layer. Cook for 2-3 minutes per side, undisturbed, until golden. If the halloumi releases liquid, let it evaporate before turning. Add the honey and turn the pieces to coat, then remove from the pan and chop into bite-sized pieces.
Once the grains are cooked, transfer to a serving bowl. Drizzle over 2-3 tablespoons of the lemon vinaigrette and toss to coat.
Add the halloumi, cherries, mint and pistachios, then top with half of the pickled onions. Toss gently to combine.
Serve with the remaining vinaigrette and pickled onions on the side for spooning over.