Preheat oven to 375 degrees F.
In a greased skillet or large baking dish, place corned beef. Score 3 slices into top of flesh and insert 1 tbsp of butter into each score. Cover surface with whole mustard seed.
Cover tightly with foil and bake for 1 hour.
While beef bakes, cut potatoes and add to large mixing bowl. Chop onion and add to mixing bowl with potatoes. Mix in oil and all spices (parsley, rosemary, thyme, basil, garlic powder, salt and pepper).
When beef cooks for 1 hour, remove from oven and add potatoes to fill the rest of baking dish. Make sure to spread out potatoes in baking dish around the beef.
Bake for another hour. Leave foil on beef for 40 min, then remove foil for last 20 min.
Take cabbage and cut into 4 large pieces. Boil a large pot of water, and once brought to a boil, add cabbage. Boil for about 7-8 min. Poke cabbage with a fork or knife to see if soft, and if so remove from boiling water. If not, continue to boil until soft.
Drain cabbage in a strainer and set aside to cool.
Once beef and potatoes are finished baking, serve in skillet/baking dish, with a side of boiled cabbage and extra mustard.