1 cup finely chopped green cabbage
3/4 cup grated carrot (about 1 medium carrot)
1/2 cup finely chopped red bell pepper
1 small green chile (Thai, serrano or jalapeño), stemmed, seeded and finely chopped
1/4 cup lightly packed fresh cilantro leaves, finely chopped
3/4 cup (6 ounces) plain whole milk Greek-style yogurt
1/2 teaspoon fine sea salt, plus more to taste
1/4 teaspoon freshly ground black pepper
8 slices white sandwich bread
4 teaspoons butter (preferably salted)
1 teaspoon black mustard seeds
Ketchup or Indian chile sauce, for serving (optional)
In a large bowl, toss together the cabbage, carrot, bell pepper, chile and cilantro. Stir in the yogurt, salt and pepper until thoroughly combined. Taste, and season with more salt, if needed.
Place 4 slices of bread on your work surface and divide the yogurt mixture evenly among them. Top the sandwiches with the remaining slices of bread.
Heat a medium frying pan or griddle over medium heat until hot. Add 1 teaspoon butter and 1/2 teaspoon mustard seeds. Let them sizzle for a few seconds, then place two sandwiches in the pan (on the mustard seeds) and cook until golden and crispy on the bottom, 1 to 2 minutes. Turn them over, add another 1 teaspoon butter, and cook until golden on that side, 1 to 2 minutes.
Transfer the sandwiches (and any stray mustard seeds) to a plate and repeat with the remaining sandwiches. Serve them whole or cut into triangles, with ketchup or chile sauce, if desired.