In a large saucepan over a medium heat, warm a little oil then add the onions and cook while stirring until soft, about 4 to 6 minutes. Add the bacon and cook for a further 5 minutes.
Add the lentils, tomatoes and potatoes followed by the water and crumble in the stock cubes. Allow to simmer uncovered for about 20 to 30 minutes until the lentils are tender, but not mushy.
In a separate saucepan over a medium heat, melt the butter then add the flour and cook while stirring until golden brown. Add a small amount of the hot liquid from the stew and whisk until smooth. Pour the roux into the stew and stir to thicken.
When the stew is lovely and bubbly and all the potatoes are tender it is ready to serve.
A roux is a mixture of equal butter and plain flour that is mixed together to thicken a dish. I made mine ridiculously simple by melting a tablespoon of butter in the microwave and adding a tablespoon of plain flour and then mixing with a fork. When it's combined, add it a bit at a time to make sure it dissolves into the stew instead of sitting lumpy on top.