Cut the Challah bread into slices about 3/4 inch thick.
In a large mixing bowl, whisk eggs, milk, nutmeg, cinnamon, salt and vanilla extract together to form batter.
Cut the nectarines and plums in half and remove the pits. Sprinkle brown sugar on top of the fruit before grilling.
Add 1 tbsp. of butter to a cast iron skillet and heat over medium-high heat.
Soak each slice of bread in the batter for about 10 seconds, ensuring to coat on both sides. Add two slices of soaked bread to the cast iron skillet and cook for 2 to 3 minutes on each side, or until golden brown. Tip: in between each round of toast, wipe the cast iron skillet clean with paper towel so that the next round of toast doesn't get too brown or burn. Add 1 tbsp. of butter per each round of 2 slices of bread.
Heat a grill pan on medium-high to high heat. Add fruit face side down to grill pan and cook for a few minutes per side, or until fruit has grill marks and is cooked all the way through.
When you're ready to assemble your French toast, spread a thick layer of mascarpone cheese to the top of one slice of French toast. Repeat to add as many layers as you want.
Add a dollop of mascarpone cheese to the top of the stack, top with blueberries and raspberries and/or grilled fruit, sprinkle with powdered sugar and pour syrup, to taste. Serve and enjoy!
To serve grilled fruit as a side, cut into quarters or leave as halves, crumble Feta cheese over top, drizzle with fig glaze and serve.