Chop the chicken into chunks and if you have a "cook and sear" slow cooker then place it on the hob with some olive oil and brown the chicken.  Or just sear in a non-stick skillet just until golden brown.  This step is optional, I just think that browned chicken brings so much extra flavour to slow cooker meals that can tend to be bland if everything is just added raw.

Once the chicken is browned now you can add in the carrots, celery, onion, sliced garlic, rice and thyme leaves. 

Make the chicken or vegetable stock using the stock cube and about 1.25l of boiling water and cook on a high heat setting for about 3 hours or 5 hours on low. The rice will be tender by then and starting to soak up some of the broth so your soup will be very thick.

Once the rice is tender and the chicken is cooked through turn off the slow cooker. Pour in the cream and add in the spinach. Give everything a really good stir and replace the lid back on the slow cooker to wilt the spinach.

Meanwhile, quarter the mushrooms and heat a nonstick pan on high heat. Add the olive oil to the pan, add in the mushrooms and sear until the mushrooms are golden. Once they are golden add in the butter and the chopped rosemary and sauté until the butter starts to brown. This will only take about 1 minute. Once the mushrooms are golden, add them to the slow cooker along with the browned butter and rosemary. Stir them in and serve with soup with chunks of crusty bread.