Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper for easy cleanup.
Slice the zucchini into half-moons and the red bell pepper into bite-sized strips.
Place the vegetables on one side of the baking sheet, drizzle with olive oil, and toss with half of the sea salt, black pepper, and garlic powder.
Pat the salmon fillet dry with a paper towel and place it on the other side of the baking sheet.
In a small bowl, finely chop the parsley and dill, then mix them with the Dijon mustard to create a thick herb paste.
Spread the herb-mustard mixture evenly over the top of the salmon fillet, then season the fish with the remaining salt, pepper, and garlic powder.
Roast in the oven for 12-15 minutes, or until the salmon flakes easily with a fork and the vegetables are tender with a slight char.
Remove from the oven and squeeze the fresh lemon juice over both the salmon and the roasted vegetables before serving.