In a saucepan over medium heat, add the butter and allow to warm. Then add the celery and onions. Reduce the heat to medium-low and cook, stirring with a spoon, for 15 minutes. Once cooked, remove the pan from the stovetop.
Preheat the oven to 375 degrees F.
Put the Texas toast cubes in a 12-by-18-inch roasting pan. Add the celery mixture, stock, sage, parsley, garlic, salt and pepper. Gently mix and allow to rest for 20 to 30 minutes. During the rest, stir gently again with a spoon, ensuring not to break the bread apart. After resting and marinating the bread, cover with foil and bake until set, 30 to 45 minutes. Once cooked, remove from the oven and serve.