Preheat oven to 375°F (190°C) and lightly grease a loaf pan.
Mix flaxseed and water in a small bowl. Set aside to thicken while you question your life choices.
Heat olive oil in a pan. Sauté onion, celery, carrot, and garlic until soft and fragrant, around 7–8 minutes.
Add tomato paste and soy sauce, stirring for a minute. Remove from heat like the multitasking kitchen goddess you are.
In a large bowl, combine lentils, sautéed mixture, oats, bread crumbs, seasonings, and flax egg.
Pulse mixture in a food processor or mash it with a fork. Keep texture slightly chunky for that rustic, homemade vibe.
Transfer into loaf pan, pressing down firmly. Smooth the top with a spatula like you’re icing cake (but sadder).
Spread ketchup on top like a culinary diva. Bake for 50 minutes until firm and browned.
Cool for 10 minutes before slicing, unless you're emotionally unstable and need instant lentil comfort.