In large skillet, melt butter over medium heat. Add onion cook until softened, about 8 minutes. 1/2 cup butter,1 large onion.
In a very large bowl, place cubed bread and pour cooked onion mixture over bread. 12 cups cubed, day-old bread.
In medium bowl, whisk together chicken broth, beaten eggs, sage, thyme, poultry seasoning, salt, and pepper. 3 cups chicken broth,3 large eggs,2 teaspoons dried sage,1 teaspoon dried thyme,1 teaspoon poultry seasoning,1 teaspoon salt,1/2 teaspoon black pepper.
Pour broth mixture over bread mixture and stir gently until well combined and moistened.
Spray 7-quart crockpot with cooking spray. Transfer dressing mixture to crockpot and spread evenly.
Cover and cook on low 4 hours until set and edges are golden brown.
Let rest 10 minutes before serving.
Notes
Don't use fresh bread! It needs to be day-old or slightly stale so it can absorb the liquid without turning to mush. My Mom always tore her loaf of bread up and let it sit out in a covered bowl overnight, but if I forget to, I just dry it out in the oven. Cube the bread and place on a large sheet pan, don't crowd it, bake at 250°F for 30 minutes to dry it out.