Place a 2 quart sauce pot on medium heat. After about 20 seconds (or whenever heated), add butter. When butter melts, add onions, garlic, and jalapeño and cook for 1 minute. Add mushrooms and cook for another 1 minute.
Deglaze with beer, then cook for 30 seconds. Add blacking season, Worcestershire, salt, and cream. Bring to a simmer, then add oysters and cook 45 seconds.
Finish with a squeeze of lemon and scallion. Split between 4 bowls served with toasty bread.