Cut the pomegranate into quarters, then press back the outer skin to push the seeds out into a bowl.
Using a sharp knife, cut a slice off the top and bottom of the grapefruit, then remove the peel and pith, cutting downward. Cut out the segments from between the membranes, then add to the pomegranate.
Finely grate the rind of half an orange and set aside. Using a sharp knife, cut a slice off the top and bottom of both oranges, then remove the peel and pith, cutting downward and being careful to retain the shape of the oranges. Slice horizontally into slices, then cut into quarters. Add the oranges to the pomegranate and grapefruit and stir to mix well.
Combine the orange rind with the lime rind, garlic, vinegar, lime juice, sugar and mustard in a nonmetallic bowl. Season to taste, then whisk in the oil.
Cut the avocado in half lengthwise and twist the 2 halves in opposite directions to separate. Stab the pit with the point of a sharp knife and lift out. Carefully peel off the skin, dice the flesh, and toss in lime juice to prevent discoloration.
Place the lettuce in a serving bowl. Top with the fruit and avocado then pour over the dressing and toss. Garnish with the onion and serve immediately.