In a large pot, add the chopped onion, and minced garlic. Cook in a large knob of butter until tender.
Open the can of corned beef and carefully remove it from the can. It may come out in one piece or be in chunks, depending on the brand. If not in chuncks, cut into 1-2 cm sized cubes.
Once the onion is tender and the garlic is fragrant, add the olive oil and chopped cabbage.
While the vegetables are cooking, you can prepare the mustard sauce. In a small bowl, combine the Dijon mustard, sweetner, white vinegar, and black pepper. Mix well to create a smooth sauce.
Once the vegetables are tender, add the canned corned beef to the pot. You can break it into chunks and mix it in with the vegetables. Allow the mixture to cook for another 5-10 minutes to heat the corned beef thoroughly. The corned beef will break up quite a bit but that's ok.
Add the mustard sauce over the corned beef and vegetables just befre the end and ensure that the sauce is warmed up sufficiently (do a taste test!).
Remove the pot from heat, and serve the corned beef and vegetable mixture on plates. We like it served in bowles.