It is widely available already made up, but usually with vegetables. It’s almost always in some kind of pouch
https://www.japancentre.com/en/produ...egetables-mild

For dry curry, it’s along the lines of this kind of recipe:
https://justhungry.com/japanese-dry-curry
But instead of the step where you add the spices you just combine together a bit of shredded/crumbled curry and some liquid on one side of the pan and, once heated, stir it through the cooked mixed rice. I don’t use raisins or tomato paste for this.

We really like pickled beetroot as an accompaniment. We’ve found quite a few Japanese ex-pats have gotten a taste for pickled beetroot with their curries too. My kid likes to melt a thin slice of Gouda or Edam on the top of a “wet” curry too. Occasionally I add peeled grated apple to a wet curry just before I add the shredded roux, it’s a good way to use apples that aren’t quite at their best or starting to shrivel.

Vegetables that can be kept in the freezer and go well with this kind of wet curry are: lotus root, “sato imo” known as Edo potatoes in the U.K. (Renkon/lotus root and sato imo should be available cheaply already peeled and prepared in the Japanese market freezers) as well as spinach, butternut squash and green beans. I tend to add fresh mushrooms, but they would be fine if you have them sliced and frozen. I use fresh onions and carrots too.

Occasionally I make a wet curry to eat with thick udon noodles instead of rice. For this I add dashi instead of water (you can get dashi concentrate in bottles, no need to make it from scratch) and add less of the curry roux so that it’s a little thinner than normal.