To make the bread bowl and croutons:
1. Preheat the oven to 350° F and line a baking sheet with parchment paper.
2. Use a sharp serrated knife to cut a hole in the top of the sourdough boule. Scoop the bread from inside of the loaf, leaving about 1” of thickness around the outside of the boule to form a wall. Set the bread bowl aside.
3. Tear the bread that has been scooped out of the loaf into large, craggy pieces and place them on the baking sheet. Drizzle them with olive oil and sprinkle with salt and then toss them with your hands to coat.
4. Bake for 15-17 minutes, until deeply golden brown and crispy. Set aside to cool.
To make the dip:
1. Preheat the oven to 350° F. Slice the head of garlic in half crosswise and drizzle the cut sides with 1 tbsp olive oil. Wrap the garlic tightly in aluminum foil and roast for 30-35 minutes, until the garlic is soft and golden. Let cool and then squeeze the roasted garlic out of the papery skins.
2. While the garlic is roasting, heat a large cast-iron skillet over medium heat. Add 1 tbsp olive oil and 2 tbsp of butter to the skillet. When the oil and butter are hot, add the thinly sliced onion along with a large pinch of salt. Sauté for about 5 minutes, until the edges of the onion begin to soften and brown, and then reduce the heat to low. Cook, stirring frequently, for 40-45 minutes, until the onions are deeply caramelized and very soft.
3. Place the remaining 4 tbsp of butter in a large Dutch oven set over medium heat. When the butter is bubbling, add the nutmeg, red pepper flakes, drained artichoke hearts, and chopped spinach along with another large pinch of salt. Cook, stirring regularly, for about 10-15 minutes, until spinach is wilted and most of the liquid has cooked off.
4. Stir in the caramelized onions and roasted garlic, followed by cream cheese and heavy cream. Cook, stirring constantly until cream cheese is completely melted, and then add the shredded parmesan and salt. Let cook for 2 minutes more until melted and then taste and add salt if needed.
5. Transfer the hot spinach dip to the prepared bread bowl and serve immediately with croutons and chips, if desired.