Rinse strip loin to remove residue and season with salt. Combine horseradish and black pepper in a small bowl to make a paste. Place loin in roasting pan and coat entirely with paste. Roast until the meat reaches an internal temperature of 140 to 142 degrees F as measured with an instant-read thermometer, about 30 to 40 minutes, then remove from oven and let rest to carryover cook to about 150 degrees for medium-rare, or cook for whatever length of time is needed (usually specified on your meat thermometer) for your desired level of doneness.
For the sauce, heat grapeseed oil in a large saucepan over medium heat. When it begins to shimmer, add the beef trimmings and cook for about 10 minutes until you see that the fat is rendered from the meat. You should pour off all but 2 tablespoons fat or, if necessary, supplement with grapeseed oil if you do not have 2 tablespoons in the pan. Add garlic and onion and gently saute until the onion turns translucent. Pour beer into the pan and cook over medium heat until most of the alcohol has evaporated, then stir in the beef base (which has been diluted in water) and tomato paste. Return to a boil, lower the heat to medium-low and allow to reduce by half. Strain into a bowl and gradually whisk in cubed butter. Nest bowl in another bowl filled with hot water and cover to keep warm.
For the vegetable medley, blanch carrots and haricots verts for 2 minutes in salted water and drain well. Heat the grapeseed oil over medium-high heat in a large saute pan. When it begins to shimmer, add shallot, peppers, carrots and green beans and season with salt and pepper. Saute until the shallot turns translucent and the peppers begin to soften. Remove to a bowl and cover until needed. Reserve the saute pan.
For the rissole, boil the potatoes until soft, and drain well.
For the onion rings, heat oil in the reserved saute pan over medium-high heat. Combine flour, cornstarch, baking powder and salt in a small bowl and pour the beer in a stream while whisking constantly. Dip the onion slices into the batter and fry until golden. Drain on paper towels and keep warm. Discard oil and reserve the saute pan.
Return to the rissole. In the same pan saute pan, heat 1 tablespoon of the grapeseed oil over medium high heat in a large skillet, reserving the other tablespoon. When it begins to shimmer, saute the white onion until tender. Transfer the onion to the potatoes which have been boiled for the rissole, reserving the saute pan. Season potatoes, to taste, with salt and pepper, and mash together. Fold in the scallions and crab trying to avoid breaking up the crab. Allow to sit until cool enough to handle, then use 3-inch circle cutters as molds and form the potato/crab mixture into circles. In the same pan, heat additional grapeseed oil for the rissole over medium-high heat and sear the potato cakes leaving undisturbed for the first 2 to 3 minutes on each side to let the caramelization process begin and to prevent the cake from breaking apart. When you see the edges of the cake beginning to turn golden brown, flip and sear the other side. Then cook further until the cake is firm. Remove from heat and keep warm.
Slice roast sirloin. Place a rissole circle on serving plate. Place sliced beef on top and spoon demi-glace over. Top with onion rings. Arrange some vegetable medley around.