Cook the rice: add 1 cup uncooked rice to a small pot along with 1 1/4 cups water and 1/2 teaspoon of salt. Cover the pot, bring to a boil, then lower the heat to the lowest and cook for 12 minutes. Take the pot off the heat and leave to rest for 10 minutes. Open the pot and fluff the rice with a fork.
In the meantime, make the sauce: in a large bowl, whisk together soy sauce, water, honey, vinegar, red pepper flakes, and corn starch.
Chop the salmon: Remove the skin from the salmon and cut it into 1-inch (2 cm) cubes. Add the salmon to the sauce, stir together, and left to marinate while you're preparing other ingredients.
Prepare the vegetables: Cut the broccoli into very small florets, slice the mushrooms, slice the green onion, finely chop garlic, and ginger.
Stir-fry: heat the oil over medium-high heat, add the broccoli and mushrooms. Cook for a couple of minutes until browned and softened. Lower the heat to low, add the white and light green parts of green onions (reserve the dark green parts for garnish), garlic, and ginger. Cook for a minute. Transfer everything to a plate.
Cook salmon: Increase the heat to high and add all the salmon to the pan in an even layer (without the sauce). Cook until browned on the bottom then turn over and cook until cooked through (about 1-2 minutes). Make sure to not overcook it.
Assemble: Reduce the heat to low. Add the sauce and bring to a boil. Add the vegetables. Toss everything together.
Garnish with reserved green parts of green onions and serve with rice.