Skin and bone the salmon steak. Cut into 8 cubes, each one approximately 1 1/4 inches square. Season well with salt and pepper. Place in a mixing bowl, and add the tawny port. Stir in the tarragon leaves, reserving a few for garnish. Cover and marinate in the refrigerator for 2 to 6 hours (try to marinate it as long as possible).
When ready to cook, prepare a charcoal fire. While the fire is getting hot, prepare the sauce. Remove the salmon cubes from the marinade and reserve cubes. Pour the marinade into a small saucepan. Add the shallot and beef stock. Bring to a boil over high heat, skimming off any foam that forms on the surface. Boil the liquid until it is reduced to a glaze. Reduce the heat to low, and begin whisking in the cold butter, 1 tablespoon at a time. (Make sure that the sauce doesn't "break". When all the butter is incorporated, you'll have a smooth, medium-thick, tan-colored sauce.) Strain it into a clean saucepan and keep warm over very low heat, stirring occasionally.
When the sauce is almost ready, grill the reserved salmon cubes: First dry them very well, then toss them with the vegetable oil to prevent sticking. Place them on the grill and cook, turning them with tongs, until charred on the outside, still rosy-pink on the inside, about 5 minutes. (Alternatively, you could place the oiled cubes under a preheated broiler, turning, until just done.)
Place each salmon cube on a small plate. Sprinkle with coarse salt. Drizzle each cube with a half teaspoon of the sauce (reserve the leftover sauce for another use). Sprinkle each cube with a few flecks of the reserved tarragon leaves, finely minced.
Copyright David Rosengarten, The Dean & DeLuca Cookbook, Random House, 1996