Pre-heat oven to 425F. Dice the sweet potatoes. Put on a large sheet pan, lined with parchment paper.
Drizzle with olive oil and sprinkle with 1 tsp each of cumin and coriander. Add some salt to taste all over.
Roast sweet potatoes in the oven for about 25 minutes, turning half-way.
While the sweet potatoes cook, dice the cucumber and cherry tomatoes. Set aside. Drain the black beans.
Make the avocado dressing: blend all ingredients in a blender or food processor until emulsified. If too thick, you can thin out with a little water or olive oil.
Once sweet potatoes are cooked through, remove from the oven and make up the bowls: divide the sweet potatoes, cucumber, tomatoes, black beans and spinach between 4 bowls. Top with feta cheese, lemon wedges, some of the avocado dressing and fresh ground pepper.