Chop the cauliflower into chunks and place them in your food processor. Pulse until the cauliflower becomes the size of rice. You might need to do it in 2 batches.
Transfer the cauliflower rice to a pan over medium-low heat. Sauté for 10 minutes until it releases most of its water and softens. Mix in the soy sauce and rice vinegar.
Transfer to a bowl and add the softened cream cheese. Mix until combined. Transfer to the freezer for 10 minutes.
Meanwhile, empty the salmon can into a bowl. Chop the scallion and add most of it to the salmon, reserving some for garnish. Add the mayonnaise and mix until combined.
Scoop 1/3 cup of the cauliflower mixture and place it on a piece of plastic wrap. Flatten it, then make a well in the middle. Add one tablespoon of salmon into the well.
Join the edges of the plastic wrap to wrap the cauliflower rice over the salmon, forming a ball. Then, shape the ball into a triangle. Unwrap the rice triangle and adjust the shape as needed.
Cut each nori sheet into 3 equal strips. Place the rice triangle into the center of a strip of nori. Fold the strips over the rice.
Repeat with the remaining rice and salmon until you have 6 Keto onigiris. Garnish with chopped scallion and sesame seeds. Serve immediately or keep in the fridge.