Dice the 2 small onions.
Heat 2 T olive oil in a large skillet over medium-high heat.
Add the diced onions and cook, stirring occasionally, until nicely browned, about 10 minutes (adjust heat as needed to prevent burning).
While the onions cook, break apart the frozen cauliflower rice (both 12 oz. packages) inside their bags or in a bowl so there are no large clumps.
When the onions are browned, add the cauliflower rice and the remaining 1 T olive oil to the skillet. Season generously with salt and fresh ground black pepper to taste and increase heat to high.
Cook, stirring or turning often, until the liquid has cooked off and the cauliflower is lightly browned and cooked to your liking, about 8–9 minutes.
Remove from heat, sprinkle generously with Sumac, and serve hot.