Preheat the oven to 375 degrees.
Add the cubed potato's to the large pot with salted water, bring to a boil and cook until the potato's are soft - about 15 minutes. Drain and return the cooked potato's to the pot. Mash with a potato masher - it is ok to leave little chunks.
Add the dried lentils to the smaller pot with the veggie or no-beef broth. Bring to a boil, cover and reduce to a simmer. Cook until tender - about 10 minutes. If you are making the mushroom gravy, save any of the remaining broth.
While the potato's and lentils are cooking, add the oil to the fry pan and sauté the onions and celery for 3 to 5 minutes. Add the garlic, black beans and chopped walnuts and sauté for another 3 minutes. Now add the tomato paste and cook for a minute or 2. Then add all the spices, soy sauce, balsamic vinegar - cook for a couple of minutes and remove from the heat.
Transfer the cooked lentils and the veggie mixture to the pot with the mashed potato's and mix well. Now add the oats. The mixture should hold together, but not be too dry. You can always add a few more oats or veggie broth to get it to the right texture.
Line the bread loaf pan with parchment paper and add the mixture. You can press the mixture firmly into the loaf pan.
Bake at 375 degrees for 45 to 50 minutes or until the top turns a nice, deep brown.
I recommend leaving the lentil loaf in the loaf pan and putting it in the refrigerator to firm up. This will take about 30 minutes.