Heat a large saute pan over medium-high heat. Add butter; heat until melted and sizzling. Add apples, sprinkle with 1 tablespoon sugar, and toss. Cook, tossing, until caramelized and tender but not mushy, adding sugar 1 tablespoon at a time. Adjust heat as necessary. Season with a pinch of salt and a few drops of lemon juice. Spread on a rimmed baking sheet to cool. Refrigerate up to 1 day before assembly.