In a medium saucepan, combine rinsed quinoa, lentils, vegetable broth, and water. Bring to a boil, then reduce heat to low, cover, and simmer for 20 minutes or until quinoa is fluffy and lentils are tender. Drain any excess liquid if necessary.

While the grains cook, heat 1 tablespoon olive oil in a large skillet over medium heat. Add diced onion and cook until translucent, about 4 minutes. Add minced garlic and cook for 30 seconds until fragrant. Add chopped spinach in batches, stirring until wilted, about 2-3 minutes. Season with salt, pepper, thyme, and smoked paprika.

Preheat oven to 375°F (190°C). In a large bowl, combine the cooked quinoa and lentils with the sautéed spinach mixture. Stir in half of the cheddar cheese and all of the Parmesan. Mix well and adjust seasoning. Transfer the mixture to a greased 9x9-inch baking dish or similar casserole dish.

Sprinkle the remaining cheddar cheese evenly over the top. If using, mix breadcrumbs with melted butter and scatter over the cheese. Bake for 20-25 minutes until the cheese is melted and bubbly and the top is golden brown. Let rest for 5 minutes before serving.

Scoop a generous portion onto a white plate. Serve warm, garnished with fresh herbs like parsley or chives if desired.