Place the soy sauce, water, mirin, brown sugar, granulated sugar, garlic, ginger and onion powder in a small sauce pan and cook on medium heat, whisking until sugar has dissolved. Allow to cool.
Place the salmon bites in a dish and pour over the prepared sauce. Cover and refrigerate 2 hours, or at least 30 minutes.
Preheat oven to 450º F. Lightly coat a large sheet pan with cooking spray.
Place the salmon bites on the prepared pan, reserving the extra marinade. Bake at 450º F for 12 minutes.
While the salmon cooks, add the corn starch to 1/2 tablespoon of water and mix to combine.
Pour the slurry into pan with the reserved teriyaki sauce and stir.
Simmer on medium high heat until thickened. Remove from heat and reserve until ready to assemble the bowls.