Step 1

In a medium bowl, toss the chicken with ½ teaspoon each salt and pepper; set aside. In a large Dutch oven over medium-high, melt 2 tablespoons of the butter. Add the onion and cook, stirring occasionally, until golden brown, about 9 minutes. Push the onion to the edges of the pot, add the chicken to the center and cook, stirring just once or twice, until the chicken is no longer pink on the exterior, 2 to 3 minutes. Reduce to medium, then stir the onion into the chicken. Add the garlic, ginger and remaining 3 tablespoons butter, then cook, stirring, until the butter melts. Add the flour and cook, stirring constantly, until lightly browned, 2 to 3 minutes; some of the flour will stick to the bottom of the pot. Add the curry powder and continue to cook, stirring constantly, until fragrant and toasted, 2 to 3 minutes.

Step 2

Working in two additions, add 1 cup water while stirring and scraping the bottom of the pot to loosen the browned bits, then cook, stirring, until the mixture is smooth and thick, about 2 minutes. Stir in another 1¼ cups water and bring to a simmer. Add the potatoes, carrot and bell pepper, then cook, uncovered and stirring occasionally and scraping the bottom of the pot, until a skewer inserted into the potatoes and carrots meets no resistance, about 20 minutes; adjust the heat as needed to maintain a simmer.

Step 3

Stir in the soy sauce, mirin and ½ teaspoon pepper. Cook, stirring often, until the curry is thick enough to lightly coat the chicken and vegetables, 2 to 4 minutes. Remove from the heat, then taste and season with salt and pepper.