Place a rack in upper third of oven and preheat to 375°.
Toss radishes, potatoes, olive oil, salt, and pepper on a large rimmed baking sheet until vegetables are evenly coated. Arrange in a single layer and roast until potatoes are just fork-tender and starting to brown, 45–55 minutes.
Meanwhile, whisk Dijon mustard, maple syrup, garlic powder, a pinch of kosher salt, and a pinch of freshly ground pepper in a small bowl.
Season both sides of salmon fillet with salt and pepper. Push radishes and potatoes to sides of baking sheet; place salmon in the center. Transfer half of mustard mixture to another small bowl; set aside. Brush remaining mustard mixture over salmon; bake until almost cooked through and opaque, 15–17 minutes.
Heat broiler. Broil salmon and vegetables just until salmon is deeply browned in spots, 3–8 minutes.
Whisk juice of lemon into reserved mustard mixture and drizzle evenly over salmon and vegetables. Top with dill sprigs and serve with lemon wedges if desired.
