In a small bowl, combine tomatoes, olive oil, salt, and black pepper, tossing gently to coat.
In a medium saucepan, melt butter over medium heat. Add flour; cook for 2 to 3 minutes, whisking constantly. Gradually add beer or ale and chicken broth, whisking until smooth. Cook, whisking constantly, until mixture thickens, 2 to 3 minutes more. Reduce heat to low; add cheese, dry mustard, Worcestershire sauce, and cayenne pepper. Stir until cheese is melted and smooth.
To assemble, place a tomato slice on each Melba round. Spoon cheese mixture onto tomato slices. Garnish with parsley, if desired.
Recipe by Victoria at https://victoriamag.com/welsh-rarebit-croustades-with-plum-tomatoes-recipe/