Melt 1 Tbsp butter in 12“ skillet, add onions, salt, pepper and cook until onions soft and golden, 15-20 min. Stir in garlic and thyme and cook until fragrant, about 30 sec. Off heat stir in olives and 2 Tbsp basil; set aside.
Meanwhile, toss squash with 2 Tbsp salt in large bowl. Microwave covered for 4 min, stir, cook 4 min more (8 total). Drain in colander and cool slightly. Arrange slices on triple layer of paper towels and squeeze with triple layer to remove moisture.
Combine panko, Parmesan, and 1 tsp pepper in bowl. Spray bottom and sides of 13×9 baking pan. Evenly coat bottom with 6 Tbsp of panko mixture. Combine the remaining panko with the 4 Tbsp melted butter and mix well.
Heat oven to 450 deg.
Arrange half the squash in prepared pan and season with pepper to taste. Sprinkle 1/4 cup panko evenly over squash, spread onion mixture over top, arrange remaining squash over top, season with pepper to taste. Cover with Aluminum foil and bake until just tender, about 20 min.
Remove from oven, discard foil, sprinkle remaining panko over top. Bake uncovered until bubbling around edges and crumbs are golden brown, about 10-15 min. Cool for 15 min., sprinkle remaining basil, Serve.