Place lentils in a pot and pour in enough water to cover by 2 inches; add onion powder, celery salt, and garlic salt. Cook lentils, stirring occasionally, over medium heat until tender, about 40 minutes. Cool lentils slightly and add to a food processor; blend until smooth.
Preheat oven to 350 degrees F (175 degrees C). Grease a 9x5-inch loaf pan.
Mix lentils, green bell pepper, celery, carrots, and garlic in a large bowl. Add parsley, brown rice, chia seeds, lemon zest, lemon juice, oregano, mint, salt, and pepper to lentil mixture; mix well. Pour lentil mixture into the prepared loaf pan.
Bake in the preheated oven until cooked through, about 1 1/2 hours. Cool slightly before slicing, about 15 minutes.
