Make the balsamic glaze: combine 500 milliliters balsamic vinegar and 1/2 cup of the granulated sugar in a medium-high sided saucepan and bring to a rolling boil over medium–high heat.
Reduce heat to maintain a steady boil and simmer, stirring occasionally, until the mixture is reduced by about 80% and becomes thick and syrupy, about 15–20 minutes.
Taste the glaze when it is about halfway reduced; if it is too sharp, stir in some or all of the remaining sugar (up to the full 1 cup total) to balance the flavor. The glaze will thicken further as it cools.
Preheat the oven to 375°F (use convection if available). Line a baking sheet with aluminum foil and lightly spray or oil it.
Arrange the salmon fillets skin-side down on the prepared baking sheet and nestle the broccoli florets and whole baby portobello mushrooms around the fish. Drizzle the vegetables and salmon with 3 tablespoons olive oil, then season everything evenly with kosher salt and freshly ground black pepper.
When the glaze is ready, spoon about 1/4 cup of the balsamic glaze over the salmon and drizzle another 1/4 cup over the vegetables, or adjust to taste.
Bake in the preheated oven until the salmon is cooked to your liking: about 8–12 minutes for 3–4 individual fillets or about 12–15 minutes for one large fillet. Do not overcook; salmon will continue to carry over-cook slightly after removal.
Optional: after removing from the oven, drizzle additional balsamic glaze over the salmon and vegetables to taste, then serve immediately.
