Heat a medium saucepan over medium-high heat until hot. Add oil and swirl the pan to distribute the oil. You want to make sure you have enough oil to cover the bottom of the pan in a nice layer. Cut one small onion in half and then each half into thin julienned slices. Add the julienned onions to the hot pan. They should sizzle when they hit the pan. Add a pinch of salt to them and stir well. Cook until they are just barely browned, they should still look mostly white. You want to preserve some of the raw onion flavors.
Add tomatoes that have been diced into a large size, about 1” cubes. Allow the mixture to simmer for about 2-3 minutes to allow the tomatoes to start to break down.
Next, add the two cans of sardines. You should be sure to buy a brand of sardines that says “in tomato sauce” as this forms the bulk of your sauce. At this point, you should have turned the heat down to medium or low and allow it to cook at a slower pace. This is to allow the heat to be gentle enough just to heat the sardines through until hot. Drizzle the entire pan with the fish sauce. Taste adding more, if desired.
Top sardines with scallions.
Once the sardines are hot, prepare the cauliflower rice. I like to use frozen precut cauliflower rice because it is super simple. Heat cauliflower rice according to directions. Divide rice up among the bowls and place a few sardines on the side of the rice with plenty of tomato sauce to go with it. Serve like this with fish sauce on the side to drizzle more over the bowls, if desired.