Preheat oven to 400°F/200°C. Let the 1/2 cup of frozen corn sit out to thaw.
Thoroughly wash the potatoes and cut them into 1-inch cubes. Arrange them on a baking sheet lined with parchment paper or silicone baking mat. Sprinkle with sea salt and black pepper. Bake for 25-30 minutes, turning once half way through.
While potatoes are cooking, cook rice according to package instructions.
Fluff rice with a fork and fold in corn.
Spoon the rice mix into 1/4 of a large serving bowl or platter, then add potatoes, lettuce, and avocado in the other sections of the bowl. Place tomatoes in the center and sliced onion over the lettuce.
Add all dressing ingredients to a blender and blend until mixture is well combined and at your desired consistency; add more water if needed. Drizzle over salad. Serve immediately.