Cut the squash in half and lay face up on on a tray. Drizzle 1 T of olive oil over the top. Sprinkle salt and pepper over both halves. Flip over and add 1/4 cup water to the tray. Bake for 40 minutes or until you can easily pierce with a knife. Remove & let cool for 5 minutes.
While the squash is cooking, slice the onion into long thin slices. Salute in 1 T olive oil for 8 minutes. Add balsamic vinegar and let cook for 1 minutes. Onions will absorb it very quickly and look almost black! Remove from heat.
Build the sandwich. Lay spinach down first, add 2 slices of squash, spread brie on top of warm squash so it starts to melt. Add balsamic onions on top.
Mash 1 avocado, add salt pepper, and garlic. Spread on top piece of bread for each sandwich.
Spray the top of the sandwich with olive oil so it gets crispy when you press the sandwich. I used my mini George Foreman grill. Each sandwich took 2-3 minutes.