Preheat oven to 400 F. Line one large baking sheet with parchment. Line a second baking sheet with foil then place a wire rack over it. Set aside.
Remove & discard stems & gills from mushrooms then slice in 1/2-inch slices. Spray one end of the wire rack (over the baking pan) with cooking spray. Place mushroom slices on it & drizzle with balsamic vinegar, oil & sprinkle with garlic salt.
Spray a small piece of foil paper. Lay drained red roasted peppers on it & place it on the other half of the mushroom baking sheet.
Peel & slice Butternut squash. Arrange on one half of the parchment lined baking sheet. Brush slices with oil & sprinkle with salt & pepper.
Slice onion & place on the other half of the parchment lined sheet. Drizzle with oil & sprinkle with salt & pepper.
Place the two baking sheets in the oven. Roast the mushrooms & peppers about 30 minutes. Roast the squash & onions 45-50 minutes.
Basil Aioli
Combine mayonnaise, garlic, basil & lemon juice in a small bowl & whisk together. Season with salt & pepper to taste.
Assembly
Slice cheese. Spread all 8 slices of bread with basil aioli. Layer 1/2 of the bread slices with a slice of provolone cheese, onions, squash, peppers & Portobello mushrooms. Top each with another slice of cheese & remaining bread slices.
Spray a double-sided grill with cooking spray, cook the sandwiches until cheese has melted , the sandwiches are hot in the center & the bread is golden brown, 4-5 minutes.