Heat the vegetable oil in a large skillet. Add the mustard seeds and cumin seeds and cook over moderate heat, stirring frequently, for 1 minute. Add the onion and jalapeños, season with salt and cook, stirring occasionally, until the onion is softened but not browned, about 8 minutes. Stir in the cayenne pepper and the yogurt. Add the toasted bread and stir gently to coat with the dressing. Stir in the tomatoes, cover and cook until the bread is softened, 4 to 5 minutes. Gently stir in the cucumber and lime juice and season with salt. Spoon the salad onto individual plates and serve.