Pour beer and water into a large pot or casserole. Add salt and stir to dissolve. Bring mixture to the boil over medium high heat.
In a large frying pan, heat vegetable oil over high heat. Sear ribs on both sides. Place on kitchen roll to briefly drain. Add seared ribs to the beer/water mixture. Add more water as needed to cover the ribs. Cover pot and simmer for 2 hours.
While the ribs are simmering, melt the butter in a saucepan and saute the onions and garlic until onions are translucent. Stir in tomatoes, ketchup, treacle, Worcestershire, mustard, salt, hot sauce and vinegar. Bring to a slow boil, stirring constantly. Reduce heat to low and leave it simmering until the ribs are finished.
Preheat oven to 180 C / Gas 4.
Drain ribs and arrange them in a shallow roasting tin, or tins. Ladle the sauce over the ribs evenly, slice each slice of lemon in half, and distribute the half slices on top of the sauce. Cover with aluminium foil loosely, and place in the oven for 15 minutes, and then uncover for the last 10, a total of 25 minutes oven time. Remove peel from lemon slices, discard peel, returning the insides of the lemon to sauce on top of the ribs, and serve ribs covering each with sauce on the plate.
To make on the BBQ...
Make the recipe as stated up until the last step. Preheat the BBQ to high heat. Place ribs in foil on the cooking grate, and cook 20 mins. Remove ribs from foil, and place directly on the cooking grate. Continue cooking for 5 to 10 minutes, basting frequently with the sauce, until ribs are slightly caramelised as desired.