For Veal: Season veal stew meat with 1 tsp salt and 1 tsp pepper. Heat 2 tbsp oil in rondo. Brown 1/2 of veal; remove from pan. Repeat with remaining veal and oil. Pour off drippings. Set aside.
Melt 1/4 cup butter in rondo; add 3 cups wild mushrooms, 2 cups onions and 1 cup celery. Sauté until onions are golden brown. Add red wine; bring to a boil. Cook until reduced to au sec. Add veal stock and russet potatoes. Return veal to pan; bring to a boil. Reduce heat; cover tightly and braise in 325°F oven 2 hours or until veal is fork-tender.
Strain solids; set aside. Reduce liquid over high heat to 4 cups. Combine 1/4 cup flour and 1/4 cup cold water; stir slurry into reduced liquid. Bring to a boil; cook until thickened. Stir in 1/4 to 1/2 tsp pepper sauce. Return solids to pan. Season with salt.
For Buttermilk Biscuits: Combine 1 lb 4-1/2 oz (4 cups) all-purpose flour, 4 tsp baking powder, 1-1/2 tsp salt and 1 tsp baking soda in mixer bowl on low speed 10 seconds or until blended. Add 6 oz (3/4 cup) cold unsalted butter, cut into pieces; mix on low speed 2 to 3 minutes or until mixture is crumbly. Add 1-1/2 cups buttermilk, 4 oz (1 cup) shredded Cheddar cheese and 4 oz (1 cup) shredded Monterey Jack cheese; mix on low speed about 10 seconds or until a soft dough forms. Do not overmix. Knead dough gently on floured surface until smooth. Roll out 3/4 inch thick. Cut into 3-1/4-inch diameter circles. Place on parchment-lined sheet pans. Bake in 425°F oven 14 to 16 minutes.
For Avocado Salsa: In large bowl combine 1 lb (3 cups) diced avocados, 7 oz (1-1/4 cups) diced tomatoes and 3-1/2 oz (3/4 cup) chopped white onion. Season with kosher salt, freshly ground black pepper and 1/2 tsp Tabasco Pepper Sauce. Cover and refrigerate.
Per order: Split 1 biscuit horizontally in half. Ladle 1 cup veal stew onto bottom biscuit half; top with 1/3 cup avocado salsa. Lean biscuit top against veal mixture. Garnish with celery leaves.