Preheat the oven to 400. Line a baking sheet with parchment paper.
Sautee onion over medium heat, until translucent.
In food processor, pulse onions with beets, walnuts, spices, salt, and olive oil, until color and consistency are almost uniform, using a spatula to scrape the sides of the bowl, every so often. Transfer mixture to a bowl.
With damp hands, lightly roll the mixture into golfball sized croquettes. Space them evenly on baking sheet. Bake for about 20-30 minutes, or until they've started to turn slightly golden. Croquettes will firm up as they cool. Serve in lettuce shells and pretend that they are precious pearls in an oyster shell!