Sweet Potato, Spinach & Caramelized Onion Tart with Balsamic Reduction

sheet puff pastry (pareve, frozen, thawed)

olive oil (extra-virgin, divided)

small sweet potatoes (about 4 ounces each, peeled and cut into 1/4" thick rounds)

medium onion (about 8 ounces, peeled, halved, and very thinly sliced)

garlic (peeled and finely chopped)

spinach (frozen, chopped)

balsamic vinegar

honey

salt (sea)

black pepper (coarsely ground)