Place an oven-proof dish in the oven and preheat to 425 degrees.
Heat oil in a pan over medium-high heat. Add half the onion and saute for a minute, until golden but not brown. Brown the pork chops in the pan for a minute or so on each side, seasoning with salt and pepper.
Remove the oven-proof dish from the oven and place the pork chops in it. Use about 1/4 cup of the beer to deglaze the pan; pour the juices over the pork chops and place them in the oven.
Roast the chops for 15-20 minutes, until they reach about 140 degrees internal temperature. Baste them halfway through with pan juices.
While the chops are roasting, make the beer sauce: Heat the butter and a little olive oil in a small saucepan over a medium flame. Cook the onion in the pan until translucent, then add the flour. Toast the flour-fat-onion mixture until it reaches a golden color and loses its raw, floury scent. Add the rest of the bottle of beer and bring to a boil. Season with salt and pepper, lower the flame, and cook until the liquid thickens and reduces to about half its original volume.
When the chops are cooked, remove them from the oven. Add the mustard and gherkins to the beer sauce and immediately pour it over the meat. Garnish with chopped parsley and serve.