Fill a large bowl halfway with water and a couple teaspoons of salt.
Chop the cabbage into 1" thick slices, then cut that slab in half (so you have approximately 1" thick slices, about 2-3" long).
Place cabbage in the water to soak.
Melt the butter in a large frying pan.
Add the onion, give a sprinkling of salt and pepper, and saute until softened.
Add the minced garlic and saute for about 30 seconds.
Add corned beef to the pan and stir to mix, breaking up the corned beef.
Stir in the thyme.
Drain the cabbage and add to the pan, giving a sprinkling of salt and pepper. Stir to mix.
Add the liquid mixture, stirring to mix. Cook covered for about 8 minutes.
Add the tomatoes, toss to mix, and simmer uncovered a further 4 minutes.
