In a large saucepan over medium-low heat combine 315ml stock, vegemite, lentils and barley. Simmer for 30 minutes.
Meanwhile; in a medium saucepan combine remaining 185ml stock, carrot, onion and walnuts then cook until tender; about 15 minutes.
Meanwhile; bring a large saucepan of salted water to a boil. Add potatoes and cook until tender but still firm; about 15 minutes. Drain and mash.
Combine flour and water then stir into carrot mixture; simmer until thickened. Combine carrot mixture with lentil mixture and season with salt and pepper. Pour mixture into a 1 litre casserole dish. Spoon mashed potatoes over lentil mixture.
Bake in preheated oven until lightly browned on top; about 30 minutes.
Save washing up ... instead of cooking the lentils and barley separate from the vegetables, chuck the veges in with the pulses after fifteen minutes. And add more water! Those lentils are super-thirsty! - 14 Sep 2010