Melt the butter in a saucepan over medium heat. Add in the spinach, garlic powder, ground fennel, salt, and pepper. Sauté until the spinach is wilted.
Stir in the flour and cook for 1-2 minutes. Whisk in the milk and stir until the mixture is nice and thick. Taste and adjust any of the seasonings as needed.
Spoon the filling into the mushrooms. Arrange the stuffed mushrooms on a foil-lined baking sheet.
Bake until the mushroom caps are tender (approximately 20 minutes). Remove from the oven and serve immediately.