Step 1: Preheat your oven to 375°F (190°C). Prepare an oven-safe ramekin (6-8 ounce capacity) or small baking dish.
Step 2: In a small bowl, combine the diced apple, granulated sugar, lemon juice, 1/4 tsp ground cinnamon, and nutmeg. Toss until the apples are evenly coated. Spoon the apple mixture into the prepared ramekin.
Step 3: In a small saucepan, melt the unsalted butter over medium heat. Continue cooking, stirring occasionally, until the butter foams, and the milk solids at the bottom of the pan turn golden brown and smell nutty (about 3-5 minutes). Immediately remove from heat and pour into a heat-safe bowl to prevent burning.
Step 4: Add the rolled oats, all-purpose flour, brown sugar, 1/4 tsp ground cinnamon, and a pinch of salt to the browned butter. Mix with a fork or your fingertips until a crumbly topping forms.
Step 5: Evenly sprinkle the browned butter oat topping over the apples in the ramekin.
Step 6: Place the ramekin on a small baking sheet (to catch any potential overflow) and bake for 18-22 minutes, or until the topping is golden brown and the apple filling is bubbly and tender when pierced with a knife.
Step 7: Carefully remove from the oven and let cool for a few minutes before serving. Enjoy your warm Single-Serve Apple Crisp with Browned Butter!