Using a medium saucepan, over medium heat, add the butter and allow it to melt completely.
Add the flour and whisk until smooth; cook, whisking constantly, for 4 to 5 minutes, until the roux turns a light caramel color.
Whisk in the curry powder, garam masala and ground cayenne; remove from the heat and set aside.
Using the large pot, used previously, over medium-high heat, add the neutral oil.
Add the onion and stir for 1 to 2 minutes until slightly softened and fragrant.
Add the potatoes and carrots; stir to combine.
Pour in the dashi or stock, soy sauce or tamari, sugar and ½ teaspoon of salt; stir to combine.
Bring to a boil; reduce heat to medium; cover and simmer for 10 to 12 minutes or until the vegetables are just tender.
When the vegetables are tender, add the curry roux and stir until the broth thickens; it should be the consistency of a thick soup broth (like chowder); if it is too thick, add some water to loosen.
Add the udon to the broth, along with the spinach; cook for about 2 minutes, until the noodles are just warmed through and the spinach is just wilted.
Divide the noodles and broth equally among four serving bowls; eat immediately.
