Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper and set aside.
In a large skillet over medium heat, saute onion, carrots, celery, bell pepper, and mushrooms sprinkled with the salt, until softened. Add the garlic and saute 30-60 seconds or until fragrant.
In a food processor, pulse together the lentils, cooked vegetables, tomato paste, BBQ sauce, flax, parsley, pepper, oats, and breadcrumbs until well-combined but with pieces of vegetable in tact.
Form the mixture into a ball and place on the prepared cookie sheet. Form into a “loaf” shape and bake for 35 minutes. Remove from the oven and spread the ketchup on top. Bake for another 10 minutes. Let the lentil loaf cool at least 10 minutes before serving, allowing the lentil loaf to keep formed.