Cook lentils: In a medium pot, add lentils and broth. Bring to a boil, then reduce to low and simmer for 20–25 minutes, until lentils are tender but not mushy. Drain excess liquid if necessary.
Sauté vegetables: In a large skillet, heat olive oil over medium heat. Add onion, carrots, and celery. Cook for 5–6 minutes until softened and fragrant. Stir in bell pepper, zucchini, and garlic, cooking another 3 minutes.
Combine base: Stir in diced tomatoes, tomato paste, thyme, oregano, smoked paprika, and cooked lentils. Let simmer 5–7 minutes until flavors meld and mixture thickens slightly. Season with salt and pepper.
Assemble casserole: Transfer lentil mixture to a lightly greased 9×9-inch or similar baking dish. Spread evenly. If desired, sprinkle with breadcrumb–nutritional yeast topping.
Bake: Preheat oven to 375°F (190°C). Bake uncovered for 20–25 minutes, until bubbling and the top is golden brown.
Rest & serve: Allow casserole to cool for 5 minutes before serving. The aroma will be savory and herby, with a lightly crisp top and tender, hearty interior.