Place kidneys and water to cover in a heavy saucepan; let stand 1 hour. Drain. Remove center cord and excess fat from kidneys; discard. Cut kidneys into small pieces.
Combine kidney, bouillon cube, boiling water, and onion in a medium saucepan. Bring to a boil. Reduce heat, and simmer 15 minutes.
Combine flour and cold water, stirring well to form a smooth paste. Stir into kidney mixture. Cook over medium heat, stirring constantly, until thickened and bubbly.
Stir in potatoes, carrots, salt, and pepper. Remove from heat; spoon into a 9-inch pie plate. Top with Baking Powder Biscuits. Bake at 400° for 15 minutes or until biscuits are golden brown.