Over low heat, into a small pot add the cheese, stout, mustard powder, Tabasco, and Worcestershire.
Cook and stir until the cheese has melted and all the ingredients have been incorporated.
Set aside and allow to cool.
Run the 4 slices of bread under a broiler to toast one side only.
Toss the cherry/grape tomatoes with some olive oil.
Spread the rarebit over the untoasted side, spread the tomatoes in the same pan, then, brown the bread under a broiler till bubbly and toasty brown, and the tomatoes are cooked, for approximately 1-15 mins.
Sprinkle with chopped chives and serve with roasted cherry or grape tomatoes.