For the bangers: Heat grapeseed oil over medium-high heat in a saute pan with a lid. Sear sausage on all sides. Add beer and onion to pan, turn heat down to low and braise until sausage is cooked through, about 50 minutes.
For the mash: Boil potatoes and onion until potatoes are tender and strain. Return to pot and stir in cheese, butter, and heavy cream. Mash by hand, then whip with an electric beater and keep warm.;
For the tobacco onions: Heat deep-fryer to 375 degrees F or as directed by manufacturer for similar foods.
Remove sausage to a utility platter and let rest. Strain braising liquid through a chinois and return to pan. Let reduce by two-thirds.
Make the tobacco onions, by combining flour, cornstarch, garlic powder, paprika, salt and pepper. Dust onions with mixture and deep-fry until crispy. Drain on paper towels.
Slice sausage on the bias. Spoon some mashed potatoes in the center of serving plate. Top with sausage and tobacco onions. Serve with some braising liquid.