For this bright, beautiful salad, use ready-to-eat peeled and steamed beets (which cut the cook time down to just 15 minutes), or roast your own.
Heat canola oil in a large skillet over medium-low. Add pecans, cumin, coriander, salt, black pepper, and cayenne pepper. Cook, stirring often, until pecans are toasted and spices are fragrant, 3 to 5 minutes. Remove from heat, and cool. Place mixed greens in a large bowl. Section oranges by cutting off the top and bottom ends of each, allowing them to stand upright. Working from top to bottom of orange and following curve of fruit, cut off peel, white pith, and outer membranes from orange to expose segments. Carefully cut orange segments away from membranes. Squeeze any remaining juices from outer membranes of orange over greens. Add orange segments, beets, vinegar, and the spiced pecans with oil. Sprinkle with crumbled feta cheese, and toss to coat.