Preheat the oven to 200°C (400°F) and line a baking tray with parchment paper

In a small bowl, whisk together balsamic vinegar, soy sauce, olive oil, garlic, and Dijon mustard until glossy and well combined

Arrange salmon fillets in the center of the tray and place asparagus and sliced onion around them

Drizzle half of the glaze over the vegetables, tossing lightly until coated

Spoon the remaining glaze evenly over the salmon, ensuring a shiny, even layer

Roast in the oven until the salmon flakes easily with a fork and the edges of the vegetables begin to caramelize

Remove from the oven and drizzle fresh lemon juice over everything for a bright finish

Serve immediately while warm and aromatic