In a small saucepan add soy sauce, 3 tablespoons water, dark brown sugar, ginger, garlic, and ground mustard. Whisk to combine.
Set over medium-high heat and whisk while bringing the mixture to a slight boil. Reduce heat to low.
In a small bowl whisk together cornstarch and 1 tablespoon of cold water. Add a bit at a time to the simmering sauce while whisking until the mixture has thickened slightly (you may not need all of it).
Once thick, but still pourable, remove from the heat and stir in sesame seeds and the green onions. Set aside while you cook the salmon.
Lightly season salmon filets with salt and pepper. In a 12-inch non-stick skillet add olive oil and set over medium-high heat.
Add the salmon, skin side down, to the pan.
Cook for 4-6 minutes, carefully flip the salmon and cook for another 4-6 minutes or just until tender and flaky.
Remove from the heat and pour the teriyaki sauce over the salmon. Garnish with green onions and sesame seeds.
