In a glass jar with tight-fitting lid, soak minced shallot in red wine vinegar. Let marinate for 15 minutes.
While the onions are marinating, wash and slice whole (un-cored) cabbage into ½ inch rounds.
Drizzle two large baking sheets with a little olive oil to coat bottom of each pan (about ½ tbsp per pan, then layer the cabbage slices onto the pan without overlapping the cabbage. Drizzle the tops of the cabbage with a bit more olive oil (another 1-2 tbsp divided between the pans) and sprinkle with kosher salt.
Roast the cabbage steaks at 375°F for 25-30 minutes or until the edges of the cabbage have started to brown and caramelized and the interior of the cabbage is tender when pierced with a fork.
While the steaks are roasting, finish making the shallot vinaigrette by adding the remaining ingredients to the marinated shallots. Seal glass jar and shake until vinaigrette is well mixed.
When ready, spoon the vinaigrette over each cabbage steak liberally, about 2 tablespoons per steak. Enjoy!