Meanwhile, heat 2 teaspoons oil in a small saucepan over medium-high heat. Add onion, 1 tablespoon curry powder and mustard seeds; cook, stirring, until fragrant, 1 to 2 minutes. Add tomatoes; bring to a boil. Reduce to a simmer and cook, stirring occasionally, until onion is tender, 10 minutes. Stir in 1/2 cup peas, cover; remove from heat.