Preheat the oven to 400° F and line a sheet pan with parchment paper.
In a small saucepan, whisk together the soy sauce, garlic, maple syrup and Dijon mustard. Cook over medium heat until reduced by half, about 5 minutes. It should be thick and syrupy.
Rub the salmon all over with grapeseed oil, then season with salt and pepper on both sides. Place the salmon fillets skin-side-down on the parchment-lined sheet pan, then brush lightly with the soy glaze.
Bake the salmon for 5 minutes, brush with more glaze, then continue cooking to your liking.
While the salmon cooks, prepare the topping. In a medium bowl, add the cucumber, sesame oil, lime juice, sesame seeds, scallions and a generous pinch of salt. Mix well, then add the avocado and gently toss until just combined.
When the salmon is finished, brush it with any remaining glaze, and serve with the avocado cucumber topping and your favorite rice if desired.