Sauté celery in 3 tbsp. olive oil until translucent and soft. Add salt & pepper to taste. Remove from pan and set aside. Repeat the process for the onions, allowing them to brown a bit more; add onions to the cooked celery. Sauté mushrooms until soft and fragrant; remove from heat and set aside, reserving the mushroom liquor. Sauté the sausage in a dry pan, crumbling it with a fork as it cooks; add a little olive oil if necessary. Prepare stuffing mix according to package directions. Add all sautéed ingredients and mushrooms with juice. Finely chop Summer Truffles and add to mixture. Mix well, check seasoning. Add fresh chopped parsley. Cook the stuffing in a covered pan for 30 minutes at 375° or use the stuffing to stuff a turkey or chicken before roasting. Before serving, drizzle with white truffle oil.