How to Make It
Preheat the oven to 425°. Spread the walnuts on a baking sheet and toast for about 5 minutes, until golden. Let cool, then finely chop and transfer to a small bowl. Stir in the mustard.
On a work surface, finely chop the garlic with the caraway seeds. Rub the ribs with 2 tablespoons of the olive oil, then rub the garlic mixture all over the ribs and season with salt and pepper.
In a large enameled cast-iron casserole, melt the butter in 1 tablespoon of the olive oil. Add the ribs and cook over moderate heat, turning once, until browned, about 5 minutes. Add the broth and wine and bring to a boil. Cover and braise in the oven for 45 minutes.
In a large bowl, toss the cabbage and fennel with the remaining 2 tablespoons of olive oil and season with salt and pepper. Arrange the cabbage and fennel on top of the ribs, cover and braise for about 45 minutes longer, until the ribs and vegetables are tender. Transfer the choucroute to a platter and top with the strained braising liquid. Serve with the walnut mustard.