Preheat oven to 350 degrees F (177 degrees C).
Peel and slice pears crosswise into rounds about 1/2-inch thick. Remove seeds carefully with a sharp paring knife. Using a medium to small star-shaped cookie cutter, cut into stars. Place the star-shaped pears in a large bowl, and toss all sides gently with a mixture of lemon juice (to prevent discoloration), cinnamon, and nutmeg.
In a large non-stick skillet, melt the butter over medium-high heat. Add the star-shaped pears and sauté for about 30 seconds. Add 2 tablespoons of the brown sugar and cook, agitating the skillet, until the sugar has dissolved (about 30-60 seconds). Remove from heat.
Lightly grease a dark 9- inch round baking pan with butter (or butter flavored non-stick cooking spray). Arrange the star-shaped sautéed pears into the buttered pan. Then, arrange raspberries (about 1 to 1-1/2 cups) in the empty spaces and sprinkle raspberries with a pinch of powdered sugar.
In a large bowl, beat the eggs, then whisk in milk, white sugar, and vanilla. Dip, tossing gently, the panettone cubes into the egg-milk-sugar mixture just enough to moisten. Using a slotted spoon, transfer half of the panettone cubes to the pan, arranging into a single layer over the layered fruits until no empty spaces are seen.
In a small bowl, mash with a fork 1 cup of raspberries with 1 tablespoon of powdered sugar. Then, pour and spread this berry mixture over the layered panettone cubes. Using a teaspoon, spoon mascarpone cheese over the mashed raspberries (or place cream cheese cubes over the panettone cubes).
Next, layer the remaining panettone cubes on top and pour any leftover egg-milk-sugar mixture over the layered panettone cubes, making sure that there is no empty space left. (At this point, the panettone French toast can be covered and refrigerated overnight if desired).
Sprinkle 2 tablespoon of brown sugar on top and bake the panettone French toast, uncovered, for about 35-40 minutes or until the top is golden. Allow to cool for 5 minutes on a rack.
Run a knife around the edges gently. Place a serving tray/plate over the top of the pan, and carefully flip it over to unmold the panettone French toast onto the plate. If desired, sift a bit of powdered sugar on top of the panettone French toast to give a snowy effect, and garnish with fresh mint leaves. Extra fresh berries can be served on the side.