Make the teriyaki sauce: In a large bowl, whisk together the mirin, sesame oil, garlic, ginger, ground black pepper, salt, hoisin sauce, soy sauce, rice vinegar, dark brown sugar, and water. Stir until the brown sugar is completely dissolved.
Preheat the oven to 360℉.
Add the salmon slices to the sauce bowl and toss to baste. Cover with plastic wrap and let marinate for 20 minutes at room temperature.
Transfer the marinated salmon (skin side down) to a lined baking sheet. Place the baking sheet on the middle rack of the oven and bake for 10 minutes.
In the meantime, transfer the remaining teriyaki sauce to a saucepan, stir in flour, and bring everything to a boil. Reduce heat to simmer, and stir occasionally until thickened. This should take you no more than 5 minutes.
Remove the salmon from the oven, brush with the teriyaki syrup, and return to bake for 10 minutes.
While the salmon is baking, steam the asparagus until tender.
Transfer the salmon teriyaki onto serving plates. Sprinkle chopped scallions and sesame seeds. Serve immediately with hot steamed rice.