Slice the bulb (bottom part) of the fennel into very thin slices or shave it.
Heat the oil in a pot on medium heat. Add the shallots, garlic, and fennel, sauté until golden brown.
Stir in the flour and cook for 1 minute. Then stir in the broth. Simmer for about 15 minutes. Season with salt and pepper to taste. Check the consistency, if its nice and thick stir in the milk and spinach, simmer for 2 to 3 minutes more. (if the soup is too thin whisk a little more flour with an equal amount of water and add it to the soup to thicken further).
Once the soup is at your desired consistency taste and adjust the seasoning as needed. Serve with crackers. Garnish with fennel fronds.
Notes
If you cannot find fresh fennel substitute ground fennel seed about 1 to 2 teaspoons depending on the number of portions you are making. Start with less and add as you go to taste.