Preheat oven to 375°F. Peel, core and cut each apple into thick (3/4") wedges and toss with lemon juice. Peel and slice banana crosswise in 3/4" thick slices. Add to apples. Add cinnamon, cloves, ginger and nutmeg to fruit/lemon mixture and gently toss to combine.
Melt 4 Tbsp. of the butter in a large skillet over medium-high heat. Add the fruit mixture and saute until tender - several minutes. Add the brown sugar and maple syrup, and cook until sugar is dissolved (approximately 1 minute.) Remove from heat and allow to cool for several minutes. Lightly butter a 13x9 baking dish. Pour fruit mixture into dish.
In a bowl, beat eggs, and stir in milk, vanilla and salt. Dip bread slices in mixture to moisten on both sides, then place slices on top of fruit. Cover entire casserole with bread (using small pieces to fill in gaps.) Dot top of casserole with remaining 2 tbsp butter.
(This can be prepared up to this point in advance. Cover casserole and refrigerate up to 24 hours.)
Bake the French toast uncovered until the top is golden brown - 30 to 35 minutes. Cool for 5 minutes.