Scrub russet potatoes and pat dry. Pierce each potato with a fork
Rub olive oil all over potatoes and sprinkle with 1 tablespoon of kosher salt.
Peel onion and cut in half. Coat with olive oil and season with 1/2 teaspoon salt and 1/2 teaspoon pepper.
Place potatoes and onions on a preheated grill at about 375°.
Bake potatoes and onion with the grill closed for 45 minutes to 1 hour until potatoes reach 200° internal temperature or yield easily when squeezed.
Remove potatoes and onion from grill. Cool slightly.
Cut an oval out of the top of each potato. Discard the top peel.
Scoop inside of potatoes into a mixing bowl, leaving the peel intact for a shell.
Add 1/2 teaspoon salt and pepper, granulated garlic, butter, sour cream, heavy cream and shredded cheese to the mixer bowl. Beat at medium speed until potatoes are smooth and all ingredients are well combined.
Spoon filling back into potato skin shells. Leave a bit of room in the top for the brisket. You will have some potato filling leftover. Store it in an airtight container in the refrigerator up to 4 days for another meal.
Chop the brisket and onions.
Top the potatoes with a generous amount of brisket and top with chopped onions.
Drizzle with bbq sauce.
Return to the grill at about 350° for 30- 45 minutes with the grill closed until potatoes are heated through.
Remove from grill and top with green onion tops.
Serve immediately.