Peel and chop Yukon Gold potatoes into even-sized chunks. Place them in a pot of salted water, bring to a boil over high heat. Cook for about 7 minutes or until fork-tender. Drain and return to pot. Add butter, milk, salt, and pepper, mashing until smooth.

In a medium pot, heat vegetable or grapeseed oil over medium heat until shimmering, about 1 minute. Add diced onions and carrots, sauté for 2-3 minutes or until onions are translucent.

Stir in thawed frozen corn and diced zucchini, cooking for an additional 2 minutes.

Pour in the required amount of water and add Japanese curry roux. Stir continuously until the roux is dissolved, creating a thick sauce, about 3-5 minutes.

Turn off the heat and fold in the finely chopped spinach until just wilted, about 1 minute. Give everything a final stir to combine.

To serve, spoon mashed potatoes onto a plate and top with warm Japanese curry.