In a large bowl, mix together the onion slices, red bell pepper chunks, zucchini, and summer squash pieces.
Drizzle with olive oil, balsamic vinegar, garlic, and salt, and toss to combine. Set aside while you make the whipped goat cheese.
To make the whipped goat cheese, add the goat cheese, olive oil, basil, lemon zest, and salt in a small food processor and pulse until it is a creamy consistency.
Preheat the grill to medium-high heat.
Place the veggies into a grill basket and grill on preheated grill for 15-20 minutes, until they are slightly charred and no longer crunchy.
Drizzle olive oil on the inside of the slices of your bread and grill for 1-2 minutes until they are slightly charred and warmed.
To assemble, spread the bottom slice of your bread with as much goat cheese as you'd like.
Top with veggies and top piece of bread.
Serve and enjoy!