Step 1Heat the oven to 450 degrees. Spray a large baking pan with cooking spray.
Step 2Wipe the tops of the mushrooms clean with a damp paper towel. Cut the dry woody ends off the mushroom stems then slice the mushrooms into 1-inch thick slices. Place the mushroom and squash slices in a single layer on the baking pan. Roast the vegetables until they're slightly browned, turning them halfway through, 7 to 10 minutes. Season them with salt.
Step 3Combine the mayonnaise, sun-dried tomato paste, lemon juice and minced basil in a small glass bowl. Rub the cut side of the toasted rolls with garlic. Spread the rolls with the mayonnaise mixture. Top the bottom of each roll with 1/2 cup of spinach leaves, 3 slices of the roasted portabello and half of the squash slices. Sprinkle 1/2 tablespoon of crumbled feta over the top of each sandwich. Sprinkle each sandwich with pepper and 1 tablespoon of shredded basil leaves. Cover each sandwich with the top of the rolls.