1 stick (8 tablespoons) unsalted butter, cut into small pieces
1 small head fennel (about 8 ounces), thinly sliced, fronds reserved for serving
Kosher salt and freshly ground black pepper
2 cloves garlic, thinly sliced
1 tablespoon fresh thyme leaves
1 small head green cabbage (about 2 pounds), quartered
1 1/4 cups dry white wine
2 tablespoons tomato paste
1/2 cup finely grated Parmesan, plus a 1-ounce piece of rind
Pinch crushed red pepper flakes
Preheat the oven to 350 degrees F.
Melt the butter in a large cast-iron skillet over medium-high heat. Add the fennel, 1/2 teaspoon salt and a few grinds of black pepper and cook, stirring occasionally, until the fennels is softened and just starting to turn golden brown, about 5 minutes. Add the garlic and thyme and cook, stirring, until just fragrant, about 1 minute.
Nestle the cabbage quarters, cut-sides down, in the pan and cook without disturbing until just softened and starting to caramelize, about 4 minutes. Flip with tongs and repeat on the other cut sides.
Meanwhile, whisk together the wine and tomato paste in a small bowl until combined. Add it to the skillet along with the Parmesan rind and red pepper flakes. Bring the sauce to a boil, then transfer the skillet to the oven.
Roast until the sauce is thickened and reduced by half, and the cabbage is caramelized and tender, about 40 minutes. Sprinkle with the grated Parmesan and return to the oven.
Cook just until the cheese has melted, about 3 minutes. Top with the reserved fennel fronds and serve immediately, spooning the sauce over the cabbage.