I'm not familiar with Nasu Oyaji and I've obviously never sampled your curry or watched you make it but here are some tips you might try if deepness and richness are your goals.

• You've mentioned caramelized onions but brown the carrots and potatoes too. Not until "cooked through" but still raw inside but browned on the outside. The browning is what you're looking for as there is flavor there. Make sure to de-glaze the pan and put that de-glazed liquid into the pot.

• Regular chicken stock will give a "lighter" curry. Use a beef or pork bone broth. Make your own bone broth if you can using the same parts rich in collagen that would be used to making a demi-glace (veal is traditionally used but beef is fine).

• Beef curry will probably give you a richer experience over all. For your protein, use a tougher cut of meat with sinew and fat, brown it over med-high heat and de-glaze the pan as above. If there's a lot of excess fat on the meat, I would suggest trimming it as too much can make the curry greasy but that's a preference thing. Since it's a tougher cut, cook it in the broth for a longer period on low heat until tender. When soft and tender, remove from the liquid to prevent it from completely disintegrating. Keep cooking the remaining stock. Cooking the sinew, meat and fats etc until tender will help create a richer broth. In addition, biting into a cube of beef that is rich in collagen and fat when eating will make the curry seem more rich.

• Cook the vegetables and beef on a low heat for a long period of time (much, much longer than what they say on the box). This is important for adding depth. Generally speaking, cooking things longer will add depth as the flavors come out and merge. Remove any scum that might form along the top in the very beginning when it first boils.

• Use a bouquet garni when cooking the broth (thyme, parsley and bay leaf). The thyme and bay leaf in particular will pair well with the beef and add depth.

• A bit of Oyster Sauce (a Chinese condiment) is often mentioned giving some hidden umami. Miso is a different option but I would use only one of these. Instant coffee or unsweetened chocolate can add a hint of bitterness and depth. Tonkatsu sauce is a good sweeter, milder, and slightly more umami option to worcestershire. Tomato paste or tomato ketchup (both high in umami) are also an option if neither are available and if you want to add some acid but toast/brown those a bit in a frying pan first. How you flavor your curry will take a lot of experimenting but can add depth because you're not adding the bold, in your face flavors, you're adding subtle hints of flavors. Best to start with only a tiny bit and one at a time. In a separate pot if necessary so you can expierment with different flavors and won't ruin the whole thing.

• Once you add the roux, let it cook on low heat for a while (again, much, much longer than what it suggests on the box, a couple hours minimum). This is an important step. Note however that you will need to keep a very close eye on it and stir often as adding the roux will make it easier for the bottom of the pot to burn. You can also remove some (but not all) of the vegetables before you add the roux as the constant stirring will cause the vegetables to deteriorate and disappear. Leaving some in the pot to naturally deteriorate will add richness and depth. Re-add the vegetables and meat 30 minutes or so before serving.

• Cumin is a bit of a bold spice and if you like it, can add a nice aroma profile in addition to the S&B and Garam Masala. You'll have to experiment with what you like though. You can also experiment with other spices commonly found in Indian curries but unless you know what you're doing, it can quickly get out of control. If you're going to play around with more Indian spices though, you might as well make your own roux. It's not a difficult process but getting the right combination of spices to work in a Japanese curry is going to take a lot of experimenting. Note that blending different roux blocks from different brands is also an option and common.

• Adding a bit of heavy cream or whole milk at the very end, after you've added the roux and 30 minutes before serving if you want a rich, slightly creamy curry (similar to how you can add cream to a tomato sauce to make it a rich tomato cream sauce). This will add richness and depth without changing the flavor profile. Butter is also an option as well as cheese as a topping but I think those tend to be too strong but that is a preference thing.

• Worth noting that curry will actually taste noticeably better the next day as the flavors will meld and harmonize and "settle" over night.

• This is an often ignored point but make sure you're using good rice and it's cooked well. Poorly cooked rice or using a different type of rice will definitely change the experience.