Preheat your oven to 350 degrees F. Begin by cutting the challah into ½ inch cubes. Arrange the cubes across two baking sheets, spreading them out to ensure even toasting. Place the trays in the oven for about 12 minutes, switching trays on racks halfway through cooking. The challah cubes should emerge toasted and slightly golden. Once done, remove from the oven and allow them to cool.
In a large sauté pan, heat 2 tablespoons of olive oil over medium high heat. Add the sausage to the pan and cook until browned, which should take about 5 to 7 minutes. Use a slotted spoon to transfer the sausage to a bowl and reserve for later. If you are not using sausage, refer to the note at the end of this recipe.
In the same pan, add the onions, carrots, and celery. Sauté for 5 to 6 minutes until softened and fragrant. Then, add the garlic and sauté for an additional 2 minutes until aromatic.
Pour 2 ½ cups of chicken broth into the pan, along with 1 teaspoon of salt and ½ teaspoon of black pepper. Remove from heat and reserve the remaining chicken broth for later use.
In another clean skillet, heat the remaining 2 tablespoons of olive oil over medium high heat. Add the sliced mushrooms and sauté for about 10 minutes, until they begin to brown and shrink in size. If necessary, cook the mushrooms in two batches, depending on the size of your skillet.
In a very large mixing bowl, combine the toasted challah cubes, cooked sausage, the vegetable and broth mixture, sautéed mushrooms, and fresh herbs. Stir gently to blend all ingredients, ensuring the challah is evenly moistened. Add the beaten eggs to the mixture and stir until fully incorporated. The mixture may appear dry at this point, but it will absorb the liquid as it cooks.
Spray your slow cooker with nonstick cooking spray. Pour the challah mixture into the slow cooker. Set it on high heat and cover. Cook for 30 minutes.
After 30 minutes, carefully remove the cover and stir the stuffing to redistribute the liquid throughout. Check for dryness; if it seems dry, drizzle a little more broth over the top and stir again. Replace the lid and reduce the heat to low.
Let the stuffing cook on low for 4 hours, stirring every hour to ensure it doesn't dry out. Add more broth as needed, but do so carefully to avoid making it too wet and mushy. After 4 hours, stir, taste, and adjust seasonings with more salt or pepper if desired. Switch to the warm setting until ready to serve.
For a variation, feel free to make your stuffing without sausage. Simply begin by sautéing the onions, carrots, and celery in 6 tablespoons of olive oil, then continue with the recipe as instructed, omitting the sausage entirely.
