Sauté the Aromatics: Heat a large pot or Dutch oven over medium heat.
Add cooking spray or oil, then the chopped fennel and onion, and sauté for 8 minutes until soft.
Stir in the garlic for 30 seconds, then deglaze with ½ cup of white wine, simmering for 2 minutes.
Simmer the Soup: Add the rinsed split peas, ½ tsp. fennel seeds, thyme, bay leaf, and 7 cups (1680 ml) of water.
Bring to a boil, then reduce heat, cover, and simmer for 50 minutes, stirring occasionally. Uncover, add the remaining ½ cup wine, and cook for 10 more minutes until peas are tender.
Finish and Blend: Stir in the spinach until it wilts, about 2 minutes.
Remove the pot from the heat and discard the bay leaf. Use an immersion blender to purée the soup until smooth.
Season to taste with salt and pepper.
Garnish and Serve: In a small, dry skillet, toast the remaining ½ tsp. fennel seeds for 1-2 minutes until fragrant. Ladle the soup into bowls and garnish with the toasted fennel seeds.