Add the spices to a mortar - coriander seeds, black peppercorns, black mustard seeds, fennel seeds and red pepper flakes. Grind the spices as fine as possible. Transfer to a small bowl.
1 teaspoon coriander seeds, 1 teaspoon black pepper corn, 1/4 teaspoon black mustard seeds, 1/2 teaspoon fennel seeds, 1/2 teaspoon red pepper flakes