Dry the walleye fillets between pieces of paper towels until thoroughly dried. The fish can be stored like this in the fridge until ready to cook, but do not season with salt ahead of time. Once you are prepared to cook the fish, season it with the salt on both sides.
Preheat a large nonstick skillet over high heat until very hot. Add in 2 tbsp olive oil and swirl the pan to coat. Add the salted fish skin side down and cook for 30-45 seconds or so on high heat. Turn the heat down to medium/medium-high and allow the fish to cook for several minutes; still, skin side down until very crispy, about 3-4 minutes.
Flip and cook for another 3-4 minutes. This may sound like we’re overcooking the fish, but it will not ruin the fish. You possibly will need to repeat this process with the remaining fillets.
Slice the tomatoes in half and trim the tough ends of the asparagus. Then cut the asparagus into 2” pieces. Slice the garlic. Set this all aside for the sauce. Heat another pan until hot and add 1 tbsp olive oil. Add the sliced onion and garlic and cook until translucent.
Add the halved cherry tomatoes. Cook on medium heat for 30-60 seconds.
Add in the asparagus. Bring the mixture to a simmer and cook until the tomatoes have burst and released their juices. Add in 1 tbsp fish sauce. Taste the sauce, adding more salt if desired. Place the cauliflower rice into the bowls and top with the crispy fish and a spoonful of tomato sauce. Serve immediately.