In a large skillet over medium low heat, melt 2 tablespoons of butter. Add sliced fennel, sprinkle with salt and pepper and stir to coat. Reduce heat to low, cover, and cook until fennel is softened but not brown, stirring often, about 15 minutes. Let cool at room temperature.
To cook the bacon for topping preheat the oven to 400F (200C). Line a baking sheet with parchment paper and place bacon strips on it. Bake until bacon is crispy for about 15-20 minutes. Transfer bacon on paper towels to remove the excess fat. Crumble it up in small pieces and reserve for later use.
Preheat the oven to 350 F(180 C).
Bring potatoes to a boil in a large pot with water that has been generously salted. Cook until the potatoes are perfectly tender. Drain the potatoes and return them in the hot pot on low heat and shake them around for a couple of minutes to remove the excess moisture.
Using a potato masher, mash the potatoes with ¼ cup butter, add sour cream and half and half. Season with salt and pepper. Incorporate the reserved fennel and 1 cup of gruyere cheese. Do not over mix. Transfer the potatoes to a buttered 2 quart (2 liters) )shallow baking dish.
To make the pretzel crumbs topping, add pretzel to a ziplock bag and using a rolling pin, roll it over the bag, back and forth to crush them into coarse crumbs. Transfer to a bowl, and mix in melted butter and Dijon mustard.
To complete the casserole, top the mashed potatoes with crumbled bacon, remaining ½ cup cheese and top with pretzel crumbs mix.
Bake in the oven for approximately 20 minutes or until cheese is melted and crumbs are golden brown.