Add the oil and onions to a large fry pan with a lid (or a large dutch oven) over a medium heat.
Saute the onions for 10 minutes, or until softened and browned.
Add the carrots and saute for a further 3 minutes.
Add the mushrooms, and saute for a further 3 minutes, or until the mushrooms have softened.
Stir in the garlic and the chopped rosemary, and saute for a further minute.
Stir the lamb mince into the vegetables, and cooking for a further 3 minutes, or until the meat is lightly browned.
Sprinkle the flour over the meat and vegetable mixture, stir through, and cook for a further minute. It may be necessary to scrape the bottom of the pan if the meat starts to stick.
Gently pour in the stock, stirring it through the meat and vegetables, and simmer gently until the mixture slightly thickens.
Add the remainder of the ingredients, including the drained lentils and the reserved bay leaf.
Season with salt & pepper, and bring the mixture to the boil.
Reduce the heat to low, cover and simmer for 45 minutes, stirring occasionally. If the mixture thickens too much, stir in a little water.