In a medium saucepan over low heat, melt butter. Add garlic and cook for 1 minute. Add chickpea flour and stir completely to make a roux. Cook for 2 minutes, stirring frequently. Whisk in milk slowly and cook until thickened to desired consistency. Add cheese and stir to melt. Season with salt and pepper, adding more to taste if necessary.
Preheat oven to 400 degrees.
Line a baking sheet with aluminum foil and spread out sliced fennel evenly over sheet. Season with kosher salt and ground black pepper. Roast fennel in oven for 10-13 minutes, or until it starts to turn golden.
Heat a skillet over medium-high heat and add Italian sausage. Break apart into small pieces. Stirring frequently, cook sausage until well-browned and crumbled.
Once fennel is roasted, remove from oven and set aside. Turn heat in oven up to 500 degrees.
Top prepared pizza crust with Garlic White Sauce, then roasted fennel, then crumbled Italian sausage, then mozzarella. Bake 5-10 minutes, or until edges are golden and cheese is bubbly.