Combine chopped onion, minced ginger, mustard seeds, and 1/4 cup water in a 5-quart pan.

Sauté over medium-high heat, stirring frequently, until the onion is lightly browned (5-7 minutes).

Incorporate curry powder and stir for about 30 seconds to bloom the spices.

Add diced potato, chicken broth, and chopped bell pepper to the pan, scraping the browned bits from the bottom.

Bring the mixture to a boil, then reduce heat, cover, and simmer for 10 minutes.

Cut the chicken breasts into 1-inch cubes and add them to the simmering soup.

Continue cooking for an additional 10 minutes, or until the chicken is cooked through.

In a separate bowl, whisk together a small amount of coconut milk and cornstarch until smooth.

Add the remaining coconut milk to the cornstarch mixture and stir well.

Pour the coconut milk mixture into the soup and stir to combine.

Add the frozen peas and continue to simmer for approximately 10 minutes, allowing the soup to thicken.

Ladle the chicken curry soup into bowls and garnish with fresh cilantro before serving.