Cut meat from bones of lamb and remove large chunks of fat. Chop onions. Brown meat and onions in oil and butter over high heat, adding about 1/2 teaspoon salt.
Peel and dice to about 3/4" pieces half the potatoes. Add to the meat and onions along with the fresh thyme sprigs.
Add the beer and bring to a boil. Add just enough stock to cover the solids. Peel remaining potatoes and carrots and cut into large chunks (2" pieces). Place on top of stew without submerging.
Seal pressure cooker and bring to high pressure. Cook for 20 minutes at high pressure, then release naturally, allowing to sit off heat for at least 12 minutes before opening.
Set aside whole vegetables and season stew with salt and pepper to taste. Serve with whole vegetables on top and garnish with parsley and chives.