Cook the Lentils: Add the brown lentils, red lentils, vegetable broth, and water to a large pot. Bring to a boil over high heat; reduce the heat to low, cover, and let simmer for 30 minutes.
Prep: Preheat the oven to 375F once 10-15 minutes have passed. Line a 4x8” loaf pan with parchment paper and set aside.
Aromatics: Warm the oil in a large sauté pan over medium heat. Add the onion, celery, and carrot and sauté for 5 minutes, stirring occasionally.
Seasonings: Add the mushrooms, rosemary, thyme, sage, and 1 tablespoon tamari. Sauté for 8 to 10 minutes, until almost all of the liquid has evaporated from the pan. Add the garlic and sauté for an additional 1 to 2 minutes. You want the vegetables to be as dry as possible!
Strain the Lentils: Once the lentils have finished cooking, transfer them to a fine METAL strainer (do not use a strainer with large holes) and use a spatula to “stir” the lentils and remove as much extra liquid as possible. The Red Lentils will break down and become a thick puree, while the Brown Lentils will remain relatively intact.
Combine: Transfer the lentils and cooked vegetable mixture to a large bowl. Add in the walnuts, ground flax, and remaining tablespoon of tamari. Mix until everything is evenly incorporated, then use a spatula to firmly pack the “batter” into the loaf pan. There should be a little extra batter protruding from the top of the pan – I like to round this out to create an even top of the loaf.
Bake: Bake the loaf in the oven for 50 minutes, then remove and let sit in the pan for 5 minutes before carefully transferring to a cooling rack. Let sit for at least 30 minutes before serving. You can also let the lentil loaf sit in the fridge overnight if you’d like it to be firmer.
Serve: serve warm, topped with gravy or as desired. Leftovers can be reheated in the oven or toaster oven at 300F for 20-25 minutes. Store leftovers in the fridge for up to 5 days, or pre-slice and freeze for up to 2 months.