Carefully pull one cabbage leave from the head after another.

Boil about a liter of water in a bigger pot. Once the water is boiling, put in ca. 5 cabbage leaves. Let them simmer for about 7 minutes, until they are tender and easily bendable. Take them out and let them drain (e.g. in a colander). Repeat with all remaining cabbage leaves.

For the filling: In a big bowl mix together one diced onion, three diced garlic cloves, the riced cauliflower, the diced mushrooms and the walnut pieces. Season with soy sauce, thyme and pepper.

Take one cabbage leave and put about 2 to 3 tsp.s full of the filling in the middle. Carefully fold and roll up the leave. Put the roll into the slow cooker. Repeat with all remaining cabbage leaves.

In a big pot heat up some olive oil. Add one diced onion and three diced garlic cloves. Fry until soft.

Add the diced tomatoes and let simmer for about 10 minutes, until the sauce has thickened.

Season with salt, pepper and thyme.

Pour the sauce over the cabbage rolls. Be careful not to fill too much into the slow cooker (so that it won't boil over).

Close the slow cooker and either cook the cabbage rolls on high for 3 hours or on low for 7 hours.