CARAMEL SAUCE : You only need a couple of tablespoons of this rich sauce for the tartine, but it keeps well. A tiny bit of lemon juice helps keep the sugar from crystallizing as it caramelizes but does not affect the flavor. I usually like to add a pinch of salt to caramel sauce, but because the tartine is made with salty cheese, in this case, I leave it out. Feel free to add a pinch if using the caramel for something else.

Toast the sourdough on one side until golden. Spread the caramel evenly on the toasted sides of the bread. Place the apple and cheese slices over the caramel in alternating layers, sprinkle each tartine with a pinch of flaky salt, and serve.

In a small heavy-bottomed saucepan, combine the sugar, water, and lemon juice, bring to a boil over medium-high heat, and cook, without stirring (swirl the pan occasionally for even cooking), until the sugar melts and the caramel is amber, 10 to 15 minutes. (Once the caramel begins to color, it will darken quickly, so watch carefully.)

Immediately remove the pan from the heat and add the cream; be careful, as the mixture will bubble and steam. Return the pan to medium-low heat and stir until smooth. Add the butter a few pieces at a time, stirring until melted. Cool the sauce slightly serving, warm or room temperature.

NOTE : To store the caramel, let cool to room temperature, cover, and refrigerate for up to 1 month. To reheat, microwave on high, stirring every 20 seconds, or heat in a small saucepan over medium-low heat, stirring.