Slice the onion thin, place in a bowl, add the juice of 1 lemon and set aside for 20 minutes
Quarter the green and red cabbage, remove the cores
Slice thin and place in to a large bowl
Julienne the carrots and place in to bowl
Pick the parsley leaves and place in to the bowl
Drain the onions, reserving the lemon juice, and add the onions to the bowl
Toss to mix all the vegetables through
Dress the slaw with mayonnaise, mustard, sugar, and mustard seed
Add reserved lemon juice as needed until slaw pops with brightness