Slice onions. Peel carrots and apple. Then slice the apple in cubes.
Heat pot on medium. When the pot gets hot, add oil. Then, add onions, carrots, apple and salt. Mix together and fry for 5 minutes.
Next, add ketchup, worcestershire sauce (optional), water, espresso shot, curry powder (optional), and chicken bouillon (optional). Mix together and boil for 15 minutes until carrots are soft (covered).
Use an immersion blender to blend everything together. Or place all contents in a blender and blend until sauce is smooth. If you want the sauce to be extra smooth, you can put it through a sieve.
On medium heat (in the same pot), add a block of curry and miso paste in the blended sauce. Try to add curry block and miso paste in increments and whisk thoroughly so the sauce is not clumpy.
When the sauce is mixed well, add in a slice of butter and mix well. Serve over rice and katsu for a delicious curry meal!