How to Make It
Preheat oven to 325°F. Place Italian bread cubes in a large bowl; drizzle olive oil over bread while stirring constantly. Season with 1/2 teaspoon salt and 1/4 teaspoon white pepper. Spread in a single layer on a parchment paper–lined rimmed baking sheet. Bake in preheated oven until toasted, about 30 minutes, stirring once halfway through cooking. Set 3 cups Italian bread croutons aside. (Bread will shrink during baking.) Reserve any remaining croutons for another use.
Spread challah bread cubes in a single layer on 2 parchment paper–lined rimmed baking sheets. Place 1 baking sheet on top oven rack and the other on bottom rack; bake at 325°F until challah is dry, about 15 minutes, rotating baking sheets top to bottom halfway through cooking. Set challah croutons aside.
Working in 2 batches, pulse onions, bell pepper, and celery in a food processor until finely chopped, about 3 times.
Melt butter in a large skillet over medium-high until sizzling. Add onion mixture, parsley, and sage, and cook, stirring often, until vegetables are just tender but not browned, about 15 minutes. Stir in poultry seasoning, remaining 2 teaspoons salt, and remaining 1/2 teaspoon pepper. Cook, stirring constantly, until fragrant, about 1 minute. Remove from heat.
Whisk together stock, orange juice, and eggs until thoroughly blended. Toss together Italian bread croutons, challah croutons, and onion mixture in a large bowl until combined. Gradually pour stock mixture into crouton mixture, stirring constantly, adding more stock if stuffing looks dry. Let stand 10 minutes.
Spread stuffing in an even layer on a parchment paper–lined rimmed baking sheet; cover tightly with aluminum foil. Bake at 325°F until hot, about 30 minutes. Uncover and bake until lightly browned, about 15 minutes.
You can substitute 3 cups store-bought seasoned croutons for homemade Italian bread croutons.