Place the rack in the center of the oven and preheat to 180 degrees Celsius (350 degrees Fahrenheit). Line a loaf pan with parchment paper or use a 12×23-cm (5×9-inch) silicone pan.
Cut the vegetables into pieces and coarsely chop the garlic. Heat the oil in a frying pan over medium heat. Using a food processor, separately chop the carrot and celery, then the mushrooms, then the onion and garlic. As you do so, add these vegetables to the skillet and sauté for about 7 minutes or until golden brown. Add the tamari and tomato paste and cook 1 minute more.
Meanwhile, coarsely chop the walnuts with the oats in a food processor. Place in a large bowl. In a food processor, coarsely chop the lentils and put them in the same bowl with the remaining ingredients and the vegetable mixture. Season with salt and pepper if desired and mix well. Press the lentils into the pan. Bake for 30 minutes.
Meanwhile, mix all the ingredients for the sauce in a small saucepan. Bring to a boil, then simmer 20 minutes over medium-low heat, uncovered, stirring occasionally.
After 30 minutes of cooking, spread half of the tomato sauce on the bread. Return to the oven for 20 minutes. Save the remaining sauce (freeze) for a next lentil bread recipe or add a few more spoons to the slices when serving.
Serve with a steamed green vegetable and small roasted potatoes.
