Preheat oven to 350°. Set beets in an 8-in. square pan with 1/4 in. water. Cover tightly with foil and bake until tender, 1 to 1 1/4 hours. Also roast walnuts in a pie pan until golden, 12 to 15 minutes. Let cool.
Meanwhile, toast cumin and coriander in a frying pan over medium heat until fragrant, 3 to 4 minutes. Coarsely grind with a mortar and pestle. Pour into a small bowl. Whisk in salt, pepper, honey, vinegar, and lemon zest, then oils.
Rub beets with a paper towel to remove peel. Cut into matchsticks. Combine beets, 1 1/2 tbsp. dressing, and 1/2 cup pomegranate seeds; let stand 15 minutes.
In a large bowl, toss lettuces, parsley, and walnuts with 3 tbsp. dressing. Arrange on a platter. Spoon beet mixture down center and scatter onion on top. Sprinkle with remaining pomegranate seeds. Serve remaining dressing on the side.