Preheat oven to 350°F (175°C). Lightly butter a 9x9-inch or 2-quart ceramic baking dish.

Brown the Butter: In a small saucepan over medium heat, melt 1.5 sticks of butter. Swirl pan occasionally as it foams and sizzles. After 4 to 5 minutes, golden brown flecks will appear and release a rich, nutty aroma. Immediately remove from heat and pour into a heatproof bowl to stop cooking. Let cool 10 minutes.

Prepare Apple Layer: In a large bowl, toss the sliced Honeycrisp and Granny Smith apples with lemon juice, maple syrup, 1.5 tbsp flour, cinnamon, nutmeg, and vanilla extract until evenly coated. Transfer into the prepared baking dish and spread evenly.

Assemble Crumble Streusel: In a medium bowl, stir together the rolled oats, 1 cup flour, dark brown sugar, cinnamon, and sea salt. Pour the browned butter (including all toasted amber milk solids) over the dry mixture. Toss with a fork until moist clumps and crumbly clusters form.

Scatter & Bake: Scatter the brown butter oat crumble evenly over the apples, completely blanketing the fruit. Bake at 350°F for 40 to 45 minutes, or until the apple juices are bubbling vigorously around the edges and the crumble topping is deep golden brown and crisp.

Rest & Serve: Allow the crumble to rest for 10 minutes so juices settle slightly. Spoon generous warm portions into bowls and top immediately with a scoop of vanilla bean ice cream!