Whisk together the ranch seasoning, pepperoncini brine, olive oil, and soy sauce in a shallow dish or zip-top bag.
Add the tofu and turn to coat. Refrigerate for at least 1 hour or up to overnight, turning occasionally. Reserve the marinade after removing the tofu.
Heat a grill or grill pan to medium-high heat (375 to 450°F). Oil the grates generously.
Thread the tofu onto skewers along with the pepperoncini peppers and red onion, alternating pieces.
Place the skewers on the grill and leave undisturbed until they release easily, about 3 to 4 minutes per side, until the tofu is golden and the onion is lightly charred.
Drizzle the reserved marinade over the finished skewers and serve immediately.