Preheat oven to 350 degrees.
In a small loaf pan with parchment paper and set aside. In a small bowl, combine the tomato sauce, 2 tablespoons tamari, agave, liquid smoke, and 1 tablespoon garlic. Set aside.
Heat a large skillet over medium heat, add 2 tablespoons of water or vegetable broth and 1/2 cup onions. Sauté 5 minutes. Add the portabello mushrooms, and sauté for 7 minutes. Transfer to a small bowl.
Add remaining 2 tablespoons of water or vegetable broth to the skillet and sauté the celery, carrot, remaining 2 teaspoons garlic, salt, and pepper over low heat. Cover and cook for 10 minutes, stirring occasionally.
In a food processor, pulse the rice and lentils 10 times, then transfer to a large bowl. In the food processor, pulse cooked vegetables, Worcestershire sauce, remaining tablespoon tamari, mustard, parsley, arrowroot, and 1/3 cup of the tomato topping 10 times, then add vegetable mixture to rice and lentils. Fold in reserved sautéed onions and mushrooms, breadcrumbs, and the 1/2 cup chopped onion.
Press half of the mixture into the prepared loaf pan and spread the top with half of the tomato topping. Then, press the rest of the mixture into the pan, and coat with the remaining tomato topping. Cover with parchement paper first, then cover with foil and bake for 45 minutes. Uncover and bake 15 minutes until top is browned. Remove from oven and cool for 20 minutes before serving.