3 tablespoons coriander seeds
2 tablespoons black peppercorns
1 teaspoon yellow mustard seeds
1 tablespoon packed light brown sugar
1 teaspoon smoked paprika
1 teaspoon garlic powder
1/4 teaspoon ground allspice
2 porterhouse steaks (1 1/2 inches thick; about 2 pounds each)
Kosher salt and freshly ground pepper
1 tablespoon vegetable oil, plus more for the grill
4 tablespoons unsalted butter, at room temperature
3 tablespoons spicy brown mustard
1 tablespoon chopped fresh parsley
1 tablespoon chopped fresh chives
Kosher salt and freshly ground pepper