Preheat oven to 350 degrees Fahrenheit. Spray a muffin pan or one large cake pan with nonstick cooking spray.

In a saucepan, combine apples, lemon juice, brown sugar, cinnamon, and two tablespoons water. Cook over medium-low heat until apples are very soft. The apples should break down and be almost sauce-like. Let cool.

In the bowl of an electric mixer, combine cooked apple mixture, eggs, ground almonds, sugar, cinnamon, fresh thyme, and lemon zest (if using). Mix to combine. 

Add mixture to muffin cups and sprinkle sliced almonds on top. Bake 15–20 minutes for small cakes, or 35–45 minutes for one large cake.

Combine sugar, water, and lemon juice in a saucepan and mix to combine. Place saucepan over medium-high heat and cook until mixture is amber colored. DO NOT stir pan, simply swirl pan occasionally.

Pour mixture onto a prepared pan. Drop slivered almonds into mixture and let cool until hardened like glass.

Break into shards and serve with cake.