Fullblood Wagyu Italian Sausage Bread Pudding

Ingredients

6 CUP Bread Cubes (from a rustic country loaf, crusts removed, and diced into half-inch cubes)
3 TBSP Unsalted Butter
1 Package Double 8 Wagyu Sweet Italian Sausage (cut into rings)
3 CUP Leeks (sliced using both white and light green parts)
1 1/2 LB Cremini Mushrooms (stems trimmed and sliced into quarter-inch pieces)
1/4 CUP Medium or Dry Sherry
2 TSP Kosher Salt
1 TSP Freshly Ground Black Pepper
1/3 CUP Fresh Italian Parsley (minced)
1 TBSP Fresh Tarragon Leaves (chopped)
4 Large Eggs
1 1/2 CUP Heavy Cream
1 CUP Chicken Stock
1 1/2 CUP Gruyere Cheese (graded and divided)