In a small saucepan over medium heat, combine soy sauce, water, granulated erythritol, rice vinegar, ginger, and garlic.
Stir the mixture until the erythritol dissolves.
Sprinkle xanthan gum into the sauce, whisking continuously to prevent clumps.
Bring the sauce to a simmer, then reduce heat to low and cook until the sauce thickens to a syrup-like consistency, about 5-7 minutes. Remove from heat and set aside.
Preheat your oven to 400°F (200°C).
Place salmon fillets on a baking sheet lined with parchment paper.
Brush each fillet with the teriyaki sauce, reserving some for serving.
Bake in the preheated oven for 12-15 minutes, until the salmon flakes easily with a fork.
While the salmon is baking, heat olive oil in a large skillet over medium heat.
Add the riced cauliflower to the skillet, sprinkle with salt and pepper, and sauté for 5-8 minutes, or until tender and slightly golden.
Remove from heat and keep warm.
Divide the cauliflower rice among four plates.
Place a salmon fillet on top of each rice bed.
Drizzle the remaining teriyaki sauce over the salmon fillets.
Garnish with chopped green onions.