Fresh walleye with mustard-dill sauce

Ingredients

• 1 tbsp Dijon mustard
• 1 1/2 tsp maple syrup
• 1 1/2 tsp red-wine vinegar
• 1 tbsp olive oil
• 1 tbsp chopped fresh dill
• 1/4 tsp dried oregano
• 1/8 tsp hot-red-chili-flakes
• 1/8 tsp garlic powder
• 1/8 tsp salt
• 3 180-g walleye, or other white lake fish fillets
• 1 tbsp unsalted butter
• 1 green onion , sliced
• 1 lemon , cut into 6 wedges