You can substitute apples for the pears. If you have pear brandy, by all means use it in place of the sherry and brandy.
Preheat oven to 350°.
Heat the oil in a large nonstick skillet over medium-high heat. Add pear slices, and cook, without stirring, 2 minutes or until golden brown. Carefully turn pear slices; cook for 2 minutes or until golden brown. Add onion, celeriac, and garlic; saute 3 minutes or until lightly browned. Add sherry and brandy, and cook until liquid almost evaporates. Remove from heat; cool.
Combine pear mixture, bread, and remaining ingredients in a large bowl, tossing gently. Spoon the bread mixture into a 2-quart casserole. Cover with lid, and bake at 350° for 45 minutes or until thoroughly heated.