1.Prepare the dauphinoise potatoes by slicing them thinly and layering them with butter, salt, and cream in a baking dish.
2.Season the tomahawk with salt and cook it to your desired doneness on a grill or skillet.
3.In a saucepan, melt butter and sauté chopped red onion until translucent. Add green pepper, cook briefly, then deglaze with brandy.
4.Stir in the beef broth and let it reduce slightly before adding heavy cream and mustard. Simmer until thickened.
5.Sauté portobello mushrooms and asparagus in butter until tender.
6.Serve the tomahawk topped with green pepper sauce alongside dauphinoise potatoes and sautéed vegetables.