Heat olive oil in a large, heavy saute pan or skillet over medium-high heat. Season ribs with salt & pepper to taste.
Brown short ribs on all sides. Place browned short ribs in a 6-7 quart slow cooker.
In the same pan you browned the ribs in, add the onion, garlic and mushrooms and saute over medium-low heat until onion is slightly tender (about 3-5 minutes). Deglaze the pan with the 1/2 cup water. Add the stock cubes (or tub of concentrated stock), the entire can of diced tomatoes (including liquid), the bay leaf, carrots and the thyme. Mix well.
Pour over ribs in slow cooker. Cover and cook on HI 4-6 hours, LOW 8-10 hours. Cooking times are a guide only as slow cookers do cook at different rates. Check at minimum times for each setting for doneness.
Remove ribs to serving platter, cover with foil and keep warm in a low oven (approx 200 degrees F).
Remove as much fat from the pan juices as possible (for what I do, see narrative above).
Transfer pan juices to a large skillet and reduce sauce to 1/2 over medium-high heat. This will take about 5 minutes. This step may be eliminated if you want a less-intense flavor to the sauce.
Pour sauce over ribs on serving platter. Serve with rice or over mashed potatoes.