Mix the salmon marinade ingredients. Coat the fillets and refrigerate for 30 minutes.
Boil the potatoes and carrots in salted water for 10–12 minutes, until slightly tender. Drain and cool briefly.
Toss with the onions, mushrooms, olive oil, thyme, paprika, garlic powder, salt, and pepper.
Spread the vegetables on a lined baking sheet and place the salmon on top.
Bake at 200°C for 25–30 minutes, or until the salmon reaches 145°F/63°C. Increase to 210°C for the final 5 minutes for extra browning.
For the sauce, melt the butter and lightly sauté the garlic. Stir in the flour, then gradually whisk in the milk.
Cook until thickened. Stir in the lemon juice, mustard, dill, salt, and pepper.
Serve the salmon and vegetables with the warm sauce.
