In a large saucepan over medium-low heat, combine 1¼ cups broth, lentils and barley. Simmer for 30 minutes.
Meanwhile, bring a large pot of salted water to a boil. Add potatoes and cook until tender but still firm, about 15 minutes. Drain and mash. Add milk, butter, and salt.
Meanwhile, in a medium dish, warm peas and corns in the microwave for 5 minutes.
Meanwhile, sauté the onions for 5 minutes, add flour and stir until thoroughly coated, then quickly add milk a bit at a time, stirring constantly. Melt cheese into the white sauce.
Add the lentils, barley, peas, and corn to the mixture and season with salt and pepper.
Pour mixture into a 2 quart casserole dish. Spoon mashed potatoes over mixture.
Bake in preheated oven until lightly browned on top, about 30 minutes.