Preheat the oven to 375°F. Lightly grease a loaf pan or line it with parchment paper for easier removal.
In a small bowl, stir together the ground flaxseed and water. Let it sit for 5 to 10 minutes until it thickens into a flax egg.
Heat the olive oil in a large skillet over medium heat. Add the onion, carrots, celery, and mushrooms. Cook for 8 to 10 minutes until the vegetables are softened and most of the moisture has evaporated.
Stir in the garlic, tomato paste, smoked paprika, thyme, Italian seasoning, garlic powder, onion powder, salt, and pepper. Cook for another minute until fragrant.
In a large bowl, mash the chickpeas with a potato masher, leaving some texture. Add the cooked lentils and mash lightly so the mixture still has some whole lentils for the best consistency.
Add the sautéed vegetables, soy sauce, vegan Worcestershire sauce, Dijon mustard, flax egg, oats, breadcrumbs, and parsley. Mix thoroughly until everything is evenly combined. The mixture should hold together when pressed. If it feels too wet, stir in a few extra breadcrumbs.
Transfer the mixture into the prepared loaf pan and press it down firmly into an even layer.
In a small bowl, whisk together the ketchup, brown sugar, Dijon mustard, and apple cider vinegar. Spread half of the glaze evenly over the top of the loaf.
Bake for 40 minutes.
Remove the loaf from the oven, spread the remaining glaze over the top, and return it to the oven for another 15 minutes until the glaze is caramelized and the loaf is firm.
Let the meatloaf rest for at least 10 minutes before slicing to help it hold its shape.
Serve warm with your favorite sides.