Bring a pot of salted water to a boil.

Add cabbage leaves and boil them until just tender and slightly wilted, about 1 minute.

Drain leaves and pat them dry, taking care not to tear them.

Meanwhile, melt butter in a large sauté pan over medium heat.

Stir in onion, 1/2 teaspoon of the salt and pepper.

Cook until onion turns translucent, 5 to 7 minutes.

Stir in garlic and cook until fragrant, about 2 minutes.

Transfer onion mixture to a large bowl.

Stir in pork, veal, fennel, thyme leaves, eggs, tomato paste and cooked rice.

Use a large wooden spoon to combine well.

Preheat the oven to 300°F.

To fill cabbage leaves, lay a leaf flat on a cutting board and use a paring knife to cut out the thick part of the stem.

Place a small handful of the mixture over the broadest part of the leaf (the quantity will depend on the size of the leaf; it should be full, but not bursting at the seams).

Fold in both sides to cover the filling and slightly overlap.

Roll up the leaf, starting at the stem end, into a small bundle.

Place stuffed roll seam side down in a Dutch oven or other ovenproof casserole.

Repeat until entire mixture is used, placing cabbage rolls tightly against one another and making a second row on top of the first.

Pour stock over cabbage rolls.

Add crushed tomatoes, bay leaves, thyme sprigs and remaining 1/2 teaspoon salt.

Cover and bake until cabbage is perfectly tender and filling is cooked through, about 2 hours.

For serving, spoon some of the cooking liquid over the cabbage rolls.