Slow Cooker Winter Root Vegetable Stew

1/4 cup olive oil
2 medium yellow onions, large dice
Kosher salt
1 1/4 teaspoons ground ginger
1 (3-inch) cinnamon stick
1/2 teaspoon ground coriander
1/4 teaspoon ground cumin
1/8 teaspoon cayenne pepper
Pinch saffron threads
Freshly ground black pepper
1 pound Farm potatoes, large dice
1 pound carrots (about 4 to 5 medium), peeled and large dice
1 pound rutabaga (about 4 medium), peeled and large dice
3 cups low-sodium chicken or vegetable broth
1 beet peeled and large dice
1 turnip or large radish, pealed and large dice
1 pound sweet potatoes (about 2 medium), peeled and large dice
1 (15-ounce) can chickpeas/ garbanzo beans, drained and rinsed (about 1 1/2 cups)
1/2 cup golden raisins, also known as sultanas
1 bunch spinach or tatsoi or kale, trimmed and washed (about 2 cups loosely packed)
1 1/2 tablespoons cider vinegar, plus more as needed
Heat the oil in a large frying pan over medium heat until shimmering. Add the onions and a pinch of salt and cook over medium heat until translucent, about 4 minutes. Add the ginger, cinnamon, coriander, cumin, cayenne, saffron, and a pinch of pepper and cook until fragrant, about 1 minute.
Transfer the mixture to a slow cooker, add the potatoes, carrots, rutabaga, beet, turnip, and broth, season with salt and pepper, and stir to combine. Cover and cook on high for 1 1/2 hours.

Add the, sweet potatoes, chickpeas, and raisins, season with salt, and stir to combine. Cover and continue to cook on high until a knife easily pierces the vegetables, about 2 hours more, stirring after 1 hour. Add the spinach and gently mix (do not overmix). Let sit until wilted. Gently stir in the vinegar, taste, and season with more salt, pepper, and vinegar as needed. Optional- remove half of the contents of the soup and mix with a stick blender or in your regular blender. Return the mixed portion to the unmixed portion and stir.