Prepare a grill for medium-high heat. Toss onions with oil on a rimmed baking sheet; season with salt and black pepper. Grill until lightly charred and wilted, about 3 minutes (extra-large bulbs will take a little longer). Let cool; finely chop.
Mix onions, shichimi togarashi, and red pepper flakes into butter in a small bowl; season with salt and black pepper.
Butter can be made 1 week ahead. Cover and chill. Bring to room temperature before using.
