Preheat your oven to 350°F (175°C). Lightly grease a 9-inch deep-dish ceramic pie pan, cast-iron skillet, or 8x8-inch baking dish with butter.

In a large mixing bowl, combine sliced Honeycrisp and Granny Smith apples, lemon juice, brown sugar, flour (or cornstarch), cinnamon, nutmeg, and vanilla extract. Toss gently until every slice is glossy and evenly coated. Transfer into the prepared baking dish and spread evenly.

In a separate medium bowl, whisk together rolled oats, flour, brown sugar, cinnamon, and fine sea salt. Add the cold cubed butter. Use a pastry cutter or your fingertips to work the butter into the dry mixture until moist, crumbly clusters form (pea-sized and almond-sized clumps).

Evenly scatter the oat streusel mixture over the spiced apples, completely blanketing the fruit. Bake in the preheated oven for 38 to 42 minutes, until the oat topping is deeply golden brown and crisp, and the apple juices are thick and bubbling vigorously around the edges.

Remove from oven and let rest for 10 minutes so the fruit juices settle into a luscious saucy consistency. Spoon warm apple crisp into dessert bowls, crown with a generous scoop of vanilla bean ice cream, drizzle liberally with warm salted caramel sauce, and sprinkle a pinch of flaky sea salt. Serve warm!