In a very large skillet, melt together the olive oil and butter, then sauté diced onion over medium-low heat until it is translucent, about 5 to 7 minutes.
Stir in the garlic and sprinkle the flour over it all then cook for 2 minutes, stirring constantly to coat.
Add the beer, brown sugar, mustard, pepper, and Worcestershire sauce to skillet and heat until sauce thickens, stirring constantly; season with salt to taste.
Place fish pieces in the skillet and cook until fish flakes easily with a fork.
Serve and enjoy!
Note: if you don't have a sufficiently large skillet to cook all the fillets at once, you may need to cook the fish in two batches after the sauce is completed or use two pans with the sauce divided between them.