01 - Preheat the oven to 400°F (200°C) and line a baking tray with parchment paper.
02 - In a large bowl, toss sweet potato cubes and red onion slices with 1 tablespoon olive oil, ground cumin, smoked paprika, salt, and pepper until evenly coated.
03 - Arrange the vegetables in a single layer on the prepared tray and roast for 25 to 30 minutes, turning once halfway through, until sweet potatoes are golden and tender. Remove from oven and allow to cool slightly.
04 - Whisk together the remaining 2 tablespoons olive oil, balsamic vinegar, Dijon mustard, honey, salt, and pepper in a small bowl or jar until emulsified.
05 - Combine spinach, cucumber, avocado if using, and roasted vegetables in a large salad bowl.
06 - Drizzle the dressing over the salad and toss gently to combine all ingredients evenly.
07 - Sprinkle crumbled feta and toasted pumpkin seeds or walnuts on top before serving.