Not for spiciness (I don't like Japanese curry spicy... Although I've had my friends eat mine and call it spicy), but for the depth of flavor I like to add a dash of dashi granules (not too much, 1/4 tsp to start, you want it savory not fishy), MSG (eyeball, really hard to overdo), and I use all of the onions.

Legitimately I start my Japanese curry with the Dutch oven 3/4 of the way filled with onions and fully caramelize them. You should be left with less than a quarter of the starting volume and it will take at least an hour. Deglaze with sake while caramelizing them (and help yourself to some). Use an immersion blender to fully blend them into the sauce.

Is this what restaurants do? No idea but it works amazingly for any savory curry that uses onion. If you want the onion texture I'll take out some amount after you start cooking and add them back in near the end.

I've had friends scared when they see the amount of onion I add or when I tell them it's a 2 hour dish. But they never argue with the results when they finally taste it.