Preheat the oven to 475 F. Slice the cabbage in half through the stem. Cut the cabbage into 1-inch thick wedges, removing most of the thick core but leaving enough so that the wedges hold together.
Brush the cabbage wedges lightly with the olive oil and sprinkle them on both sides with the mustard seeds, salt, and pepper. Place the cabbage wedges in a single layer on a rimmed baking sheet (lined with foil or parchment if you like), and dot the cabbage with little bits of the softened butter. Cook until the wedges are charred on the edges and beginning to soften, about 15 to 20 minutes.
Sprinkle the wedges with the lemon juice and serve hot or warm.
Notes
You can switch up the seasonings for these cabbage wedges in so many ways. Instead of the mustard seeds, try caraway seeds, chili powder, or smoked paprika.
Top the warm wedges with some crisp, crumbled bacon pieces.
Serve these cabbage wedges with a small dollop of horseradish sauce.
Leftover cabbage wedges can be stored for up to a week in an airtight container. You can reheat them in a microwave or in a 350-degree oven for about 15 minutes, until warmed through.