Heat oven to 325°. Lighly oil a large baking dish. Peel off and discard any bruised or ragged outer leaves from the cabbage. Cut cabbage into eight wedges, without trimming away the heart.
Arrange wedges on their sides in the baking dish, trying for a single layer. Scatter onion and carrot slices over cabbage. Drizzle with stock or water and then oil. Season with salt, pepper, and red pepper flakes. Cover tightly with foil and put in oven.
Braise for one hour. Uncover and gently turn the wedges with tongs, keeping them as intact as possible. Add a little water if pan is drying out. Cover pan and return to oven. Bake one hour longer.
Remove foil, increase heat to 400°, and roast until vegetables begin to brown, about 15 minutes. Serve warm or at room temperature.