Place block of tofu on cutting board. With a sharp knife, cut into three equal sections lengthwise. Cut each section in half lengthwise so that you end up with six "sticks." Arrange several layers of paper towels on a plate and place tofu on top. Place several layers of paper towels on top of tofu and press down to remove excess moisture.

Arrange tofu in a container large enough to store the pieces in a single layer. Using a knife or a brush, spread mustard evenly on all sides of the tofu. Cover container and place in refrigerator to marinate for at least 3 hours or overnight.

Transfer bread crumbs to a large plate. Add pepper flakes and stir together. Thread each piece of tofu onto a skewer. Coat each piece of tofu in bread crumb mixture, pressing down gently to make sure the crumbs stick. Transfer to a plate as done and spray tofu all over with olive oil cooking spray.

Lightly spray grill with olive oil cooking spray and preheat. Grill tofu over medium heat, turning so that all four sides become browned and crispy. Remove from grill and transfer to a platter. Serve immediately. (To cook in the oven, preheat oven to 375°F and omit skewers. Use a fork to place marinated tofu into bread crumbs to coat. Place on a lightly greased baking sheet and bake for 10 minutes or until browned and crispy.)