In a medium-sized pot, heat neutral oil and 2 tablespoons of butter over low heat. When the butter has fully melted, toss in the julienned onions. Cook on low for 40 to 45 minutes until dark brown and caramelized, making sure to stir occasionally to prevent burning.
Once the onions have caramelized, add the grated carrot and apple. Cook on medium-low for 2 to 3 minutes, until the liquid from the grated apple has evaporated.
Pour in the low-sodium chicken stock and water. Bring to a boil then reduce to a simmer.
Dissolve the curry roux cubes into the pot. Add the curry powder then simmer for 5 to 10 minutes until slightly thickened.
Add the mango chutney, Bulldog fruit & vegetable sauce (or ketchup), Worcestershire sauce, dark soy sauce and white pepper. Stir and mix well.
Using an immersion blender, blend the curry until smooth. Add the remaining 2 tablespoons of butter into the curry and mix well.
The flavors of the curry will enhance as it sits so I recommend storing it in the fridge overnight and reheating it the next day.
Serve with rice and your choice of katsu!