Pre-heat oven to 375°F.
Prepare lentils by rinsing them and adding to a pot with water and a bay leaf. Bring the water to a boil, then reduce heat down and simmer covered until all the liquid is absorbed and lentils are tender (about 15-20 minutes). Remove from heat, discard the bay leaf and set the lentils aside to cool. This step can be done in advance.
In the meantime toast walnuts by spreading them onto a baking sheet and toasting in the oven for about 8-10 minutes. Remove from oven and let cool.
Heat oil over medium heat in a medium sauté pan and cook onions, garlic, carrots and celery until they've softened and become fragrant. About 5-10 minutes. Once cooked remove from stovetop to cool.
Add lentils (reserve ½ cup for later), cooked veggies, toasted walnuts, oats, tamari, tomato paste, ground flaxseed, nutritional yeast, maple syrup, apple cider vinegar, thyme, oregano, basil, cayenne pepper, ground pepper and sea salt to the food processor. Pulse until mixture is just combined and the texture is to your liking. You want the mixture to be combined enough to stick together, but you don't want it to be completely pulverized.
Stir in the reserved lentils and press mixture into parchment lined loaf pan. I used a 9" x 5" x 2½".
In a small bowl stir together glaze ingredients and spread the glaze over top of the loaf.
Bake covered with foil for 25 minutes. Uncover and bake for about 8 more minutes. Let the loaf rest 15 minutes before cutting and serving.
Store leftovers in a sealed container for up to 5 days in the fridge. Or freeze for up to 1 month.