Roasted Stuffed Onions

Name: Roasted Stuffed Onions
Ingredients: 10 medium red and yellow onions (4 lb); 1 lb sliced bacon, cut crosswise into 1-inch-wide pieces; 3 celery ribs, cut crosswise into 1/2-inch-thick slices; 1 teaspoon salt; 1 teaspoon black pepper; 3 garlic cloves, minced; 15 oz baby spinach, trimmed and coarsely chopped (14 cups); 1 (9-inch) round loaf country-style bread (1 1/4 lb), cut into 1/2-inch cubes (10 cups), lightly toasted; 2 cups salted roasted cashews (10 oz), coarsely chopped; 1 stick (1/2 cup) unsalted butter, melted; 1 1/4 cups turkey giblet stock