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
These make an amazing side dish, but I have absolutely eaten too much of this all on its own.