In a medium saucepan, boil 3 cups of water. Add the lentils, set a timer for 12 minutes, and then reduce the heat to medium. When the timer goes off, add ¼ cup of quinoa to the pot and give it a stir. Set the timer for 12 more minutes and let it cook until all the water has been absorbed.
Let the mixture cool down for a few minutes before you add it to the food processor. If there is any water left, make sure to drain it out before you add the mixture to the food processor.
While the lentils and quinoa are cooking, chop the veggies and make the flax egg. Make the flax egg by stirring together the ground flaxseed and 6 tbsp of water in a small bowl. Put it in the fridge to thicken for 10 minutes.
Preheat the oven to 375 degrees.
Pour the cooked lentils and quinoa into a large food processor cup. Add the diced onion, diced mushrooms, minced garlic, oats, nutritional yeast, sunflower seeds, tomato paste, vegan worcestershire sauce, flax egg, Italian seasonings, dried basil, dried parsley, salt, and black pepper. The food processor will be pretty full once you add all of the ingredients.
Put the lid on and pulse it on High for 1 minute, stopping every 20 seconds to use a spoon to scrape down the sides. Continue blending until all of the vegetables have broken down and everything is combined. The mixture should be a little bit chunky. Be careful not to overblend it and puree the filling. Taste and add more salt if necessary.
Line a 9 x 5-inch loaf-shaped pan with a piece of parchment paper. Let the edges stick out of the pan so you have handles to pull the loaf out of the pan. Scoop the lentil filling into the pan and use a spoon to smooth it out evenly all the way to the edges.
Bake the loaf for 30 minutes. While the vegan meatloaf is baking, make the tangy tomato glaze or mushroom gravy. To make the tomato glaze, mix together tomato paste, pure maple syrup, and balsamic vinegar in a small bowl.
After 30 minutes, remove the lentil loaf from the oven and spread half of the tomato glaze over the top. Reserve the rest for serving. Put it back into the oven to cook for 20 more minutes.
Carefully take it out of the oven and let it rest for 10 minutes. This resting time will help the vegan meatloaf to setup. To take the loaf out of the pan, grab both of the parchment paper handles, lift it up, and transfer it to a large plate or cutting board. Spread the rest of the tomato glaze on top and then slice into 1-inch wedges.
Store leftover slices in an airtight container in the fridge for 4-5 days or you can freeze them for a future meal. Freezing instructions for slices or a whole loaf are in the Notes below.