Combine vegetable oil, lemon juice, garlic, cumin, salt, and black pepper in a bowl. Add cubed lamb and marinate for about 30 minutes.
Heat a skillet over medium-high heat and brown meat on all sides, about 3 minutes per side. Remove meat and set aside. Reduce heat, add onion, and cook until soft and translucent, about 5 minutes.
Return cooked lamb to skillet. Mix in beer and cilantro. Season with bay leaf, salt, and pepper. Cover skillet and simmer over low heat for about 1 hour. Add potatoes, cover, and cook over low heat until potatoes are tender, about 30 minutes, adding water or broth if there is not enough liquid in the skillet.