In a small saucepan, over medium-high heat, whisk together the orange juice, sugar, mustard powder, ¼ cup of the vinegar, and ¼ teaspoon of the salt.
Bring to a boil, reduce heat to medium-low, and simmer, uncovered, until slightly thickened, about 8 minutes. Season with ¼ teaspoon of the pepper.
Heat broiler. Arrange the salmon on a rimmed baking sheet lined with foil. Brush generously with the glaze.
Broil until the salmon flakes easily when pricked with a fork and is the same color throughout, about 5 minutes, basting once with remaining glaze.
Place the cucumber, lettuce, mint, and orange in a bowl. Drizzle with oil and the remaining vinegar, salt, and pepper and toss. Serve with the salmon.