For the fruit-nut mixture: Mix the butter with the apples, pears, walnuts and cinnamon in a medium bowl. Refrigerate to chill briefly.
For the French toast: Meanwhile, in a large flat dish, whisk together the eggs, milk, sugar, cinnamon, nutmeg, salt and vanilla.
Make a deep slice from the side into the center of each bread slice to form a pocket. Stuff a couple spoonfuls of the fruit-nut mixture into each piece (reserve any leftover fruit-nut mixture for serving). Dip the stuffed bread in the egg mixture to moisten on both sides (but not so much that the bread falls apart).
Heat a large skillet over medium heat and melt 1 tablespoon of butter. In batches, add the stuffed bread and cook until golden brown, flipping gently, 2 to 3 minutes per side. Continue cooking the remaining bread, adding additional butter as needed.
Put the extra fruit-nut mixture in the skillet and cook a few minutes over medium heat, just to soften the fruit. Spoon over the French toast and serve with warm maple syrup.