Cover cashews with boiling water for 10 minutes (or longer) while you prep everything else. Preheat the oven to 375ºF/190ºC. Arrange one oven rack on the 2nd rack and one oven rack in the bottom (if not making the bread bowl, just one rack in the middle)
Slice off the top of the bread/boule. Using a knife, cut around the inside of the bread, but don’t cut through the sides. Scoop out the center of the bread using your hands. Use your hands to tear the bread into pieces about the size of croutons (or you can slice the bread into cubes); do the same with the top of the bread, or keep as a cover for the dip (handy if you're traveling with the dip). Make the cashew cream. Drain and rinse the cashews. Add to a high-powered blender with the water, nutritional yeast, lemon zest and juice, kosher salt, and black pepper to taste. Blend on high speed for a few minutes until thoroughly blended and all cashew bits have been pulverized. Taste, adding more salt or nutritional yeast as desired. Now add the tapioca starch and blend again (it may taste chalky but it will cook off).
In a 12-inch nonstick frying pan, heat the olive oil and butter over medium heat. Once warmed, add the shallots and garlic and cook until golden and aromatic but not browned, 2 to 3 minutes. Add the artichokes and, if using, red pepper flakes, and cook for 2 to 3 minutes. Add a few handfuls of spinach, cook until wilted, and repeat with the rest of the spinach, until most of the liquid has evaporated.
Pour in the cashew cream mixture, stirring constantly, for a minute. Cook for another 3 to 4 minutes, stirring occasionally, until no longer runny and the mixture is thickened and creamy. Turn off the heat. Stir in the parmesan, if using. Taste and season with salt and pepper, as needed (I typically don’t need to add any).
Toss the torn or cubed bread pieces with some olive oil and season with salt (as if you were making croutons). Spread out on a rimmed sheet pan.
Place the hollowed out bread bowl on a second rimmed sheet pan. Transfer the dip into the hollowed out cavity and fill to the top but don’t overfill (Note *7). Transfer the pan with the bread pieces to the bottom rack and the bread bowl dip to the upper rack. Bake both for 15 minutes. Remove the pans from the oven.
Optional: For a golden brown top, preheat your broiler for a few minutes (if you have a medium setting, use that). Add the bread bowl dip to the 2nd rack below the heat source (not directly underneath) and broil for just 1 to 2 minutes until bubbly and golden on top, keeping a close eye so the outer rim of the bread doesn’t burn.
Cool for a few minutes. Use the toasted bread pieces and/or crackers for dipping.