Preheat oven to 450 degrees
Line a baking sheet with parchment paper and place fish fillets on top. Brush them with olive oil and lemon juice, then sprinkle with some salt and pepper.
Melt the butter in a sauce pan
Once melted, add the onion and cook until they are soft and translucent. Next, add the garlic and cook until fragrant
Add the flour and cook for 1-2 minutes, whisking constantly until the color starts to become very light golden brown
Add in the milk while whisking constantly until everything is combined. Heat over medium heat and continue to stir regularly.
Once you see steam from the milk and the sauce starts to thicken, add the parsley to finish it off
Place the fish fillets in the oven and cook for 4-6 minutes (best done as your sauce is finishing so the fish fillets don't dry out)
Taste the sauce and add any extra salt or pepper you would like
Serve with a side of rice or potatoes, and a vegetable
