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.
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.
Place under the broiler, until bubbly and hot; place on plates and serve with gherkins/cornichons and salad, if desired.
Notes
I have used an herbed Havarti cheese which was delicious as well!
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