Heat 1 tablespoon of the grapeseed oil in a large cast-iron or stainless skillet over medium-high. Sprinkle salmon with 1/2 teaspoon each of the salt and black pepper. Place salmon, skin side down, in skillet; cover. Cook over medium-high until skin is crisp and salmon is almost done, about 6 minutes. Turn salmon over, and cook to desired doneness, about 3 minutes more. Transfer salmon to a plate; cover with foil to keep warm.
Wipe skillet clean with a paper towel. Heat remaining 1 tablespoon grapeseed oil in skillet over high. Add green beans; cook, stirring occasionally, until lightly charred and almost tender, about 3 minutes. Add bell pepper and onion; cook, stirring occasionally, until starting to soften, about 4 minutes. Add cannellini beans, parsley, dill, and remaining 1/4 teaspoon each salt and black pepper; stir to combine. Remove from heat.
Whisk together walnut oil, vinegar, and honey in a small bowl. Arrange bean mixture on a platter. Flake salmon into large pieces, and scatter on top of vegetables. Drizzle with dressing, and sprinkle with tarragon.