In a small saucepan over a medium heat, melt butter. Add onion and garlic, cook until fragrant about two minutes.
Reduce heat to a simmer, and add white wine and lemon juice, simmer until reduced by half about two minutes. Add blueberry puree, sugar and salt. Simmer for five minutes until reduced.
Reduce heat to low and slowly whisk butter in half a tablespoon at a time, making sure each piece is fully incorporated before adding the next. Once butter is fully incorporated, remove from heat.
Season to taste with salt and pepper.
In a large non-stick pan, heat a tablespoon of olive oil over a medium-high heat. Season mahi-mahi with salt and pepper. Cook fish, about 3-4 minutes per side until golden brown and cooked through. Serve with Blueberry Sauce.
Recipe by Cooking for Keeps at https://www.cookingforkeeps.com/mahi-mahi-with-blueberry-beurre-blanc/