Wash zucchini and yellow summer squash thoroughly, then cut into 1-inch thick rounds or chunks. Slice red onion into thick wedges.

Pat the vegetables dry with paper towels to remove excess moisture.

Preheat a cast-iron grill pan over high heat for at least 5 minutes until oil shimmers.

Drizzle high-smoke point oil over the vegetables and sprinkle with garlic powder and smoked paprika.

Place zucchini, yellow squash, and onion wedges in the hot pan without crowding. Cook undisturbed for 4 minutes.

Flip vegetables and cook for an additional 3 minutes or until tender-crisp and slightly brown.

Remove from heat, drizzle with balsamic glaze, and season with salt and pepper. Top with fresh parsley before serving.