In a large skillet, let the oil heat over medium heat.
Add in the onion, carrot, bell pepper, and garlic; cover and cook for about 5 minutes or until the veggies are softened.
Transfer veggie mixture to a 4-6 quart slowcooker.
Add in the beans, tomatoes with juice, vegetable stock, bay leaves, cumin, thyme, cayenne, salt and pepper; stir to mix.
Cover and cook on LOW for 8 hours.
Remove and throw away bay leaf; adjust seasonings to taste.
Add in lemon juice right before you are ready to serve.
To thicken soup, puree about 2 cups (or up to one-half) of soup solids by using an immersion blender right in the cooker or take out soup solids and tranfer to a blender or food processor; return puree to slow-cooker.