Preheat the oven to 350°F. 

For the basic stuffing base, spread the diced bread on a baking sheet and dry it out in the oven for 15 minutes. Transfer the bread to a large mixing bowl and set aside. Leave the oven on. 

Heat the olive oil in a medium skillet over medium heat. Add the onions, carrots, celery and garlic, season with salt and pepper and cook, stirring occasionally, until soft, about 10 minutes. Deglaze with the Cognac, then add the butter and melt. Add the stock and herbs, stir to combine, then transfer the mixture to the bowl with the bread and toss together. 

Transfer half of the stuffing base to another large mixing bowl and set both bowls aside. 

For the sausage stuffing, heat the olive oil in a medium skillet over medium-high heat. Add the sausage and brown, stirring occasionally. Transfer the sausage to one of the stuffing base bowls and toss to combine.  

For the mushroom stuffing, heat the olive oil in a medium skillet over high heat. Add the mushrooms and brown, stirring occasionally. Transfer the mushrooms to the other stuffing base bowl and toss to combine. 

Spread each stuffing onto a large rimmed baking sheet and bake until crispy and brown, 25 to 30 minutes.