1. Melt the butter in a medium saucepan over low heat, then add the milk and grated cheese, stirring until the cheese melts and becomes smooth. Add the vinegar, mustard, black pepper and gherkins; stir to combine and remove from heat.
2. Allow to cool slightly, stirring occasionally. Preheat the broiler (grill) then place the four pieces of toast on a baking sheet and divide the cheese mixture evenly between them.
3. Place under the broiler, until bubbly and hot; place on plates and serve with gherkins/cornichons and salad, if desired.