Pulse mushrooms: Add mushrooms to food processor and pulse a few times but not too fine.
Cook veggies: Heat olive oil in a skillet over medium heat. Add onions, celery, carrots, garlic, and mushrooms and cook until onions turn translucent and the liquid evaporates, about 5 minutes.
Toast tomato paste: Add tomato paste and stir for 30 seconds, then remove from heat. Set aside to cool for a few minutes.
Pulse oats and walnuts: Add oats to food processor and pulse to form a flour. Add walnuts and pulse a few times until crumbly but not finely ground. Transfer to a large mixing bowl.
Pulse tofu and lentils: Add tofu and lentils to food processor and pulse until coarsely chopped. Add to the mixing bowl with oats and walnuts.
Combine mixture: To the same mixing bowl, add the ground flax, chickpea flour, cooked veggies, and all the spices and seasonings. Mix well to combine.
Rest: Cover the bowl and rest in the fridge for at least 30 minutes to allow the mixture to thicken.
Prep oven and pan: Preheat oven to 350°F/180°C. Line a 9x5-inch loaf pan with parchment paper hanging over the sides for easy removal. Other pan sizes: Two 8x4-inch loaf pans, one 8x4-inch loaf pan plus two mini loaf pans (5.5x3-inch), or 12 muffins.
Add mixture to pan: When ready to bake, press the mixture into your prepared loaf pan(s). Pack it down tightly.
Bake and glaze: Bake for 45 minutes uncovered. Mix the glaze ingredients together and brush the glaze on top of the loaf. Bake for another 20 minutes.For mini loaves, bake 30 minutes uncovered, glaze, and bake another 10 minutes. For mufins, bake 25 minutes uncovered, glaze, and bake another 5 minutes.
Rest: Remove the meatloaf from the oven and let it rest for at least 20 minutes (10 minutes for mini loaves or muffins) before slicing.
Notes
1) Check the ingredients to ensure it's vegan-friendly. Some Worcestershire sauce contain fish/anchovies. Japanese Chuno Style Worcestershire Sauce is usually vegan-friendly.