Place Spaghetti squash cut side down on parchment lined baking sheet and bake until fork tender, about 35-45 minutes.
Make the pickled red onions. Bring water and vinegar along with garlic, salt ,sugar and whole spices to a boil, stir until salt and sugar dissolves. Place sliced onion in the pot, submerging. For a more tender onion, simmer for few minutes before chilling, or for more crunchy, turn heat off immediately and cool in the fridge.
When squash is done, scoop out its meat into a fine mesh strainer over a bowl, letting any liquid drain out for a few minutes. You want to get this DRY! Discard liquid, and place spaghetti sqaush in the bowl ( or pot) and season with salt and pepper. Add a generous amount of bbq sauce. You want this saucy! You could keep this warm in a pot on the stove over low heat, or serve immediately.
Spoon a little BBQ Spaghetti Squash over a bun, top with pickled onions and arugula, and top with the bun lid. Serve right away.
You can make the BBQ Spaghetti squash ahead and reheat when ready to assemble. These are best served warm.