Cook farro in a large pot of boiling salted water until just tender, about 15 minutes. Drain well and place in a large bowl.
Heat griddle to high. While the farro is cooking, brush the aubergine and onion slices with oil and season with salt and pepper. Griddle for 3 to 4 minutes on each side or until just cooked through. Remove from the griddle and cut into 2 1/2 cm dice.
Add the aubergine and onions to the farro along with the tomatoes and dill. Pour sherry vinegar over the farro mixture and stir to combine.
Garnish with additional dill. Best served at room temperature.
To make the sherry vinegar:
Whisk together the shallot, vinegar, mustard, salt, pepper and dill in a small bowl. Slowly whisk in the oil until emulsified.