Heat oil in skillet on medium heat until hot.
Add onion, pepper and mushrooms. Cook, stirring frequently, until vegetables are softened, about 5-6 mins.
Add garlic, tomatoes and herbs. Cook and stir until heated through.
Transfer to a large bowl.
In the skillet, melt margarine or butter and whisk in flour until smooth. Whisk in milk and bring to a low boil, stirring all the while until mixture thickens.
Whisk in cheese and stir to make a rich, velvety sauce.
Combine cooked rice with vegetables and fold in cheese.
Pour into slow cooker, cover and cook on low for 4-6 hours or high for 2-3 hours.