Step 1

Heat ¼ cup oil in a small saucepan over low heat until just warm. Add mustard seeds and cook until they start to pop and sputter, which should be a matter of seconds. Quickly remove pan from heat, add curry leaves, and toss to coat—they should crisp immediately from the residual heat. Set spiced oil aside.

Step 2

Mix together onion, green chiles, yogurt, cilantro, black pepper, and chili powder in a small bowl; season with salt. Spread yogurt mixture over 6 slices of bread and top with remaining slices to make 6 sandwiches.

Step 3

Heat 1 tsp. olive oil in a large skillet over medium heat until shimmering. Reduce heat to low and cook 2 sandwiches until undersides are crisp and golden brown, about 3 minutes. Turn, add another 1 tsp. olive oil to skillet, and cook until the second side is crisp and golden brown, about 3 minutes. Transfer sandwiches to a platter or plates; repeat with remaining sandwiches and 4 tsp. oil.

Step 4

Drizzle reserved spiced oil evenly over sandwiches. Cut in half and serve with chutney and/or ketchup alongside if desired.

Step 5

Do Ahead: Yogurt filling can be made 1 day ahead. Cover and chill.