Preheat oven to 350°F. In bowl, toss cashews with drizzle 1 tsp of olive oil and kosher salt; spread on baking sheet. Roast until golden brown, about 5 minutes. Remove from oven and let cool.
Add kimchi to small saucepan. Add enough pineapple juice to just cover kimchi. Cook, covered, on low heat until kimchi is softened, about 10 minutes. Transfer to blender and puree until smooth. Stream in 1⁄2 cup of the remaining olive oil while blending, until thickened and combined. Transfer to bowl and let cool.
Melt butter in saucepan over medium heat, stirring until the butter is lightly browned, foaming and nutty. Stir in parsley, capers and a little lemon juice, and set aside.
Place cauliflower steaks on tray and brush remaining 3 tbsp of the olive oil overtop. Season with sea salt.
Preheat barbecue to medium-high heat. Place cauliflower steaks on grill and cook without moving them around too much, until deep golden brown, about 3 to 5 minutes; flip and cook for 3 to 5 more minutes.
To serve, dollop kimchi sauce on each plate; top with 1 cauliflower steak in centre of plate and drizzle 1 tbsp of the warm brown butter overtop. Break cashews in half and sprinkle overtop. Squeeze lemon wedges over the steaks and sprinkle with a flaky finishing salt, if using.