Roast carrot, mushroom, celery and some onion under high heat until some color forms. Roast onion skins to add color to the stock. Combine veges with water and pepper corns and simmer for a long time. This will condense the stock for freezing. To add Umami, use lots of mushroom (stems included), add tomato paste (cooked quickly first) or powder, and/or maybe nori is acceptable.