Place the minced steak in a bowl with the paprika, spring onions, egg yolk and mustard and mix together with a fork until thoroughly blended. Shape into 4 patties.
Cook the burgers under a preheated high grill for 10 minutes, turning once, until well browned and cooked through.
Meanwhile, heat the oil in a large, heavy-based frying pan and cook the mushrooms over a high heat, stirring frequently, for 5 minutes until browned. Make the salsa by simply tossing the cucumber and coriander together, and season with a little pepper.
Split each roll and serve a burger in each, topped with a slice of cheese to melt, then the mushrooms and a spoonful of salsa.