Preheat oven to 350 degrees Fahrenheit.
Thinly Slice fennel, onion, and potatoes using a mandoline or handheld slicer. Can be hand-sliced if you'd like just be sure to adjust the cooking time as thicker slices will take longer to cook.
In a medium-sized pan, heat butter and olive oil and saute onion and fennel over medium-low heat for 15 minutes or until very tender. Remove from heat.
In a large bowl, combine sliced potatoes with the cream, olives, salt, pepper, and all but 2 tablespoons of each of the cheeses.
Add cooked fennel and onion to the bowl and stir to combine.
Transfer potato mixture to an 8x8" baking dish or deep pie plate and flatten the surface so all the potatoes are mostly submerged in the cream.
Combine remaining 2 tablespoons of each cheese in a small bowl with 2 tablespoons of cream. Sprinkle cheese over the top of the potatoes evenly.
Bake at 350 degrees for 45-60 minutes or until potatoes are very tender the top is golden brown and bubbly. Check for tenderness by inserting a fork or sharp knife through the potatoes in the center of the dish. If needed, tent the dish with a piece of foil to preven the top from becoming to brown.
Allow to cool for at least 15 minutes before serving.