Preheat oven to 350ºF. Arrange bread in a flat layer on a baking sheet and bake until pale golden, 8 to 10 minutes. Transfer to a serving bowl; let cool.
In a small bowl, whisk together apple cider, vinegar, mustard and 1 Tbsp. oil. In a large nonstick skillet, warm remaining 1 Tbsp. oil over medium heat. Cook sausage, turning, until golden brown on both sides, about 4 minutes total. Transfer to bowl with bread pieces. Add apples to skillet; cook, stirring, until golden and soft, about 6 minutes. Transfer to bowl with sausage and bread. Remove skillet from heat and pour in cider mixture, stirring and scraping up any browned bits.
Pour hot cider mixture into bowl with bread mixture; add onion, celery and parsley, tossing to combine well. Season with salt and pepper.