Spread the prosciutto slices on 2 rimmed baking sheets. Broil until crispy, 2 1/2 to 5 minutes per baking sheet. Set aside the prosciutto.
Add the sliced bread to the same baking sheets and broil until golden brown, 30 to 45 seconds. Flip and broil the other side until golden brown. Remove and let cool.
Add the sherry and honey to a medium bowl and whisk until smooth. Slowly drizzle in the olive oil, then season with salt and pepper. Add the arugula, basil and Pickled Onions. Toss until evenly coated.
For each sandwich, line up 3 slices of toasted bread, then spread a thin layer of Garlic Aioli on each slice. Top 2 slices with crispy prosciutto, then add some Roasted Tomatoes and arugula salad. Place 1 of the built sandwiches on top of another. Top with the remaining piece of Aioli-dressed toast for a triple-decker club. Repeat the build until you have 4 massive PLTS (save the remaining roasted tomatoes for another use)! Slice in half--or don't; just take a bite.
Pickled Red Onions:
Combine the vinegar, sugar, salt and 1/2 cup water in a medium saucepan. Bring to a boil and let the salt and sugar dissolve, about 2 minutes. Add the sliced red onion. Turn off the heat and let sit until ready to use.
Slow-Roasted Thyme Tomatoes and Roasted Garlic Aioli:
Preheat the oven to 325 degrees F. Line 2 rimmed baking sheets with foil. (For easy clean up!)
Add the tomatoes, thyme, 1/4 cup of the olive oil and some salt and pepper to a large bowl. Toss to evenly coat. Spread the tomatoes on the prepared baking sheets.
Put the head of garlic on a piece of foil, then drizzle with the remaining tablespoon olive oil and a sprinkle of salt. Seal the foil. Place the tomato pans and garlic in the oven and roast until the tomatoes are roasty and the garlic has become pure gold, 45 to 55 minutes. Allow to cool.
Remove the garlic from the foil and squeeze the flesh into a small bowl. Add the mayonnaise and stir until combined.