Mix together the water, ketchup, mustard, steak sauce and lemon juice for the sauce.
Melt the butter in a non-stick pan. When hot, add the sliced sausages and cook over high heat until the sausage slices are lightly browned in spots.
Add the onion, separating the rings. Cook, tossing often, just until the onion rings soften a bit.
Pour in the sauce. Toss to coat every piece of sausage and onion.
Turn off the heat, transfer the sausages and onions to serving bowls and serve at once. Great with bread or rice.