1. Prepare and Roast the Sweet Potatoes:
* Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper for easy cleanup.
* In a large bowl, toss the diced sweet potatoes with 2 tablespoons of olive oil, smoked paprika, cumin powder, ½ teaspoon sea salt, and ¼ teaspoon black pepper. Ensure the sweet potatoes are evenly coated.
* Spread the seasoned sweet potatoes in a single layer on the prepared baking sheet. Avoid overcrowding the pan; use two sheets if necessary to ensure even roasting.
* Roast for 25-30 minutes, flipping halfway through, until tender and slightly caramelized at the edges. Once done, remove from oven and set aside.

2. Cook the Buckwheat:
* While the sweet potatoes are roasting, rinse the buckwheat groats thoroughly under cold running water until the water runs clear. This removes any bitterness.
* In a medium saucepan, combine the rinsed buckwheat, 2 cups of vegetable broth (or water), and the optional bay leaf.
* Bring the mixture to a boil over medium-high heat. Once boiling, reduce the heat to low, cover the saucepan, and let it simmer for 15-20 minutes, or until all the liquid has been absorbed and the buckwheat is tender but still slightly chewy.
* Remove from heat, discard the bay leaf, and let it stand, covered, for 5 minutes. Fluff with a fork.

3. Sauté the Kale and Black Beans:
* While the buckwheat simmers, heat 1 tablespoon of olive oil in a large skillet or Dutch oven over medium heat.
* Add the chopped yellow onion and sauté for 5-7 minutes, until softened and translucent.
* Stir in the minced garlic and cook for another 1-2 minutes until fragrant, being careful not to burn it.
* Add the chopped kale to the skillet in batches if necessary, stirring until it begins to wilt. Pour in the ¼ cup of vegetable broth (or water) to help steam and soften the kale. Cook for about 5-7 minutes, until the kale is tender-crisp and vibrant green.
* Stir in the rinsed and drained black beans, lime juice, chili powder, dried oregano, ½ teaspoon sea salt, and ¼ teaspoon black pepper. Cook for another 3-5 minutes, allowing the flavors to meld. Taste and adjust seasonings if needed.

4. Assemble the Casserole:
* Reduce the oven temperature to 375°F (190°C) if it’s not already there from roasting the sweet potatoes.
* In a large mixing bowl (or directly in the skillet if it’s oven-safe and large enough), combine the roasted sweet potatoes, cooked buckwheat, and the kale-black bean mixture. Gently fold everything together until well combined. Try not to mash the sweet potatoes too much; you want to maintain some distinct cubes.
* Transfer the mixture to a 9×13 inch casserole dish or a similar-sized oven-safe baking dish. Spread it out evenly.

5. Add Topping and Bake:
* If using, sprinkle the chopped pecans or walnuts evenly over the top of the casserole.
* If using nutritional yeast for a vegan cheesy flavor, sprinkle it over the nuts. If using dairy cheese, sprinkle it over now.
* Bake in the preheated oven at 375°F (190°C) for 20-25 minutes, or until the casserole is heated through and the topping (if using nuts/cheese) is lightly golden and fragrant. The edges should be bubbling gently.

6. Rest and Serve:
* Once baked, remove the casserole from the oven and let it rest for 5-10 minutes before serving. This allows the flavors to settle and makes it easier to serve.
* Garnish with freshly chopped cilantro or parsley just before serving for a burst of freshness and color.