Preheat the oven to 400°F (conventional oven). Wash the sweet potato thoroughly (peel if desired) and cut into roughly ½-inch cubes. In a bowl, toss the sweet potato cubes with the oil and Mexican spice mix until evenly coated, then spread them out on a baking sheet. Roast the sweet potato cubes in the oven for about 25–30 minutes, until the edges are crispy and the insides are tender.

Meanwhile, wash the cilantro thoroughly, shake dry, and remove the thick stems. Place the leaves (with the thin stems) in a blender. Squeeze the lime and add the olive oil and garlic. Blend until smooth and creamy, with no chunks remaining. Season the dressing with salt and pepper, and, if desired, a little agave syrup to taste.

Wash the cherry tomatoes and cut them in half. Cut the avocado flesh into small cubes. Drain the black beans, rinse them thoroughly, and let them drain well.

Take the sweet potatoes out of the oven and let them cool for about 2–3 minutes. In a large salad bowl, combine the roasted sweet potato cubes, cherry tomatoes, avocado, and black beans. Crumble the feta with your fingers and stir it into the salad along with the dressing. Season with salt and pepper to taste and serve warm.