Combine the squash, 1 Tbsp olive oil, smoked paprika, ½ tsp salt, and pepper in a large bowl, mixing to coat everything evenly. Spread squash evenly on a baking sheet and roast in the oven at 400 degrees until tender, about 30 minutes.
While squash roasts, heat the remaining 1 Tbsp oil in a large pan over medium-low heat. Add the onion, and cook, stirring occasionally, until onions are caramelized and very fragrant, about 25-30 minutes. The onions should be ready about the same time the squash is done roasting.
Add the remaining ½ tsp salt and the apple cider vinegar to the pan, and cook 1 minute more. Add the spinach and cook until tender, about 3-4 minutes. Add the roasted squash to the pan. Tear the toasted bread into pieces and add to the pan. Mix everything together until well combined.
Place cheese slices on top, cover pan, and cook until cheese has melted, about 2-3 minutes. Transfer pan to the table, grab some forks and dig in.