Wash the zucchini and yellow summer squash thoroughly. Slice both vegetables into 1-inch rounds, and slice the red onion into thick wedges.

Pat the sliced squash and zucchini aggressively with paper towels to remove excess moisture.

Heat a cast-iron grill pan over medium-high heat, allowing it to warm up until it shimmers.

Add enough high-smoke point oil to coat the bottom of the grill pan.

Sprinkle garlic powder and smoked paprika over the vegetable rounds, and toss them gently in a large bowl.

Add the seasoned vegetables in a single layer to the hot grill pan and sear undisturbed for about 4 minutes.

Flip the vegetables and cook for an additional 3 minutes until tender-crisp.

Remove from heat and drizzle with balsamic glaze. Season with kosher salt and fresh black pepper, and toss with minced parsley.