Cook Lentils: In a medium saucepan, combine the rinsed lentils and vegetable broth (or water). Bring to a boil, then reduce heat to a simmer. Cook uncovered for 20–25 minutes, or until lentils are tender and liquid is absorbed. Drain any excess liquid if necessary and set aside.

Prepare Vegetables: While the lentils are cooking, heat olive oil in a large skillet over medium heat. Add the chopped onion, garlic, carrots, and celery. Sauté for 8–10 minutes until the vegetables are softened and fragrant. Remove from heat and let cool slightly.

Preheat Oven: Preheat your oven to 375°F (190°C). Line a loaf pan with parchment paper or lightly grease it to prevent sticking.

Mix Ingredients: In a large mixing bowl, combine the cooked lentils, sautéed vegetables, breadcrumbs, ground flaxseed (or chia seeds), chopped parsley, dried thyme, smoked paprika, salt, and black pepper. Mix thoroughly until all ingredients are well incorporated. If the mixture seems too dry, add a tablespoon of water or vegetable broth at a time until it holds together.

Shape the Loaf: Transfer the mixture into the prepared loaf pan, pressing it down firmly and smoothing the top to ensure it holds its shape.

Prepare Glaze: In a small bowl, whisk together the ketchup (or barbecue sauce), maple syrup (or brown sugar), and Dijon mustard until smooth. Spread this glaze evenly over the top of the loaf.

Bake: Place the loaf in the preheated oven and bake for 30 minutes. After 30 minutes, cover the loaf with aluminum foil to prevent the glaze from burning, and bake for an additional 10–15 minutes, totaling 40–45 minutes of baking time.

Cool and Serve: Once baked, remove the loaf from the oven and let it cool in the pan for at least 10 minutes. This resting time helps the loaf firm up, making it easier to slice. Carefully lift the loaf out using the parchment paper or by inverting the pan onto a serving platter. Slice and serve warm.