Preheat an outdoor grill to medium-high heat.
Toss the peppers, oil, salt, and garlic powder in a large bowl.
Grill for about 4 minutes per side, until charred all over.
Transfer to a serving platter. Drizzle vinegar all over the peppers. Scatter avocado slices, feta, and basil over top. Finish with flaky sea salt.