In a large skillet over medium heat, heat oil until shimmering. Don't overheat the pan. You don't want to burn the vegetables.
Add the onions and tomatoes. Saute for approximately 7-10 minutes until golden brown and tomatoes have softened a bit.
To the veggie skillet, add the red peppers and portobellos and saute for an additional 5-6 minutes or so. You can go longer if you like your peppers softer.
Right before serving add the garlic and the spinach and toss it all around for 1 minute.
Squeeze the lemon (minus the seeds of course) and drizzle with the balsamic scraping up all the good stuff at the bottom of the pan.
If your bun is especially doughy, remove a little of the inside of the top half of the bun. I'll leave it up to you what you do with it.
Lay three basil leaves in the top half of one bun and top with half the goat cheese.
On the bottom half of the bun, lay the vegetables. Close up the sandwich and repeat with the second sandwich. Happy late night!