Bring a large pot of water to boil. Salt it generously.
Core the cabbage. Gently peel away the leaves to expose the heart (by heart, I mean the ball of more yellowish leaves at the center which are too tightly packed to bother prying apart). Cut out that core of inner-most leaves and shred to add to the stuffing. Cut the thick ribs out of the remaining leaves (they will look like you've stolen a sliver from a pie). Set aside.
Blanch the cabbage leaves for 5 to 7 minutes. Drain, and refresh under ice-cold water. Drain and pat dry with a towel.
Break the bread into crumbs in a bowl, pour over the milk and set aside to soften. Heat the butter and olive oil in a skillet and gently fry the onion and shallot until transparent, about 5 minutes. Add the chopped cabbage, mushrooms, garlic and thyme. Cook another 5 minutes. Add the bread and cook until the milk has evaporated. Stir through the quatre-epices and season generously with salt and pepper. Add this mixture to the sausage meat in a large bowl and mix thoroughly with a fork. Make a small ball and fry it in the frying pan. Taste it to check the seasonings. Adjust as needed.
Lay a tea towel on the counter with a piece of cheesecloth or muslin large enough to wrap the cabbage in. You're going to reconstruct the cabbage, but with layers of stuffing between the leaves. So, first lay down the large outer leaves, in a circle, slightly overlapping with the prettiest side out. Spread over a layer of stuffing. Lay over another layer of leaves and repeat the action. Continue until you have run out of leaves. Pull up the edges of the cheesecloth, like a bag, and twist, as if making the head of a puppet, to shape the cabbage into a round loaf shape. Tie a string around the beard of cheesecloth where it meets the cabbage ball, to secure the package. The cabbage can be prepared to this stage in advance, refrigerated and then cooked before serving.
To cook: Steam the cabbage over water or good chicken stock (about 2 cups/500 ml of it) for 45 minutes. The flavor from the stuffed cabbage will drip into the water or stock and give it the most amazing flavor. When the cabbage is done, boil down the cooking juices and serve a spoonful around each wedge of cabbage in a soup bowl.