Add dry lentils and vegetable broth to a medium sauce pot. Bring to a boil over high heat, then reduce to a simmer and cover with a lid. Simmer until cooked through, about 15 minutes. Drain the cooked lentils.
Add roughly chopped onion and mushrooms to a food processor with a standard blade attachment. Pulse until mostly broken down into tiny pieces but not pureed.
Heat a large skillet over medium-high heat. Once hot, drizzle in olive oil. Add the mushrooms, onions, and salt. Cook 6-8 minutes, stirring occasionally.
Add walnuts to the food processor. Pulse until mostly broken down, then add to the skillet with the tomato paste. Cook another 2 minutes.
In the food processor, pulse together the cooked lentils and chickpeas until mostly broken down. They should be crumbly but not a paste. Set aside. Make oat flour by blending or processing rolled oats until fine. Combine ground flax and water in a small bowl to make a flax egg. Let rest 5 minutes before using.
To a large mixing bowl, add the mushroom mixture, lentils, chickpeas, oat flour, light brown sugar, soy sauce, Worcestershire sauce, nutritional yeast, oregano, red pepper flakes, smoked paprika, chili powder, black pepper, and flax eggs. Stir until well combined.
Transfer the filling to a lightly oiled 9x5x3 (or similarly sized) loaf pan. Form into a loaf shape. Bake at 350F for 45-50 minutes, until golden on top.
Let rest for 5 minutes before removing from the bread pan. Gently turn out onto a clean surface. Use your hands to lightly press the loaf together so that it is perfectly shaped and firm.
Mix together the ketchup, brown sugar, mustard, apple cider vinegar, onion powder, and garlic powder in a small bowl. Brush the loaf with the sauce. Slice and enjoy!
Lentils: Choose brown or green lentils. French green lentils work but will take longer to cook. Red and orange lentils do not work.
Worcestershire sauce: Choose a vegan sauce free from anchovies. Store brand Worcestershire sauce is often anchovy-free.
Leftovers and reheating: Wrap leftover lentil loaf in foil or place in an airtight container and refrigerate for 3-4 days. Reheat individual slices in the microwave in 1-2 minutes. To reheat the entire loaf, return it to a loaf pan and heat at 325F for 15-20 minutes, until warmed through.
Freezing: If freezing the entire loaf, first wrap it in foil before placing it in an airtight container. Let thaw completely in the fridge overnight before reheating. Freeze for up to 3 months.
Gluten-free option: Replace soy sauce with tamari and use gluten-free rolled oats.
Nut-free option: Walnuts can be omitted if needed.
More of a visual learner? I have a full step-by-step cooking video for Vegan Meatloaf over on my YouTube channel.