Preheat the oven to 300°. Remove 5 wide strips of zest from the orange with a vegetable peeler, and add to a small casserole dish. Add olive oil, smashed garlic cloves, ½ teaspoon each of salt and pepper, and red pepper flakes to the casserole dish. Finely grate the remaining zest and place in a small bowl. Juice the orange into a large bowl, then add shallot, mustard and 1 tablespoon of the tarragon, and set aside. Add remaining tablespoon of the tarragon and the minced garlic cloves to the small bowl with zest, and season with salt and pepper.
Put the potatoes in a large pot and cover with water by 2 inches; season generously with salt. Bring to a boil; cook until the potatoes are fork-tender, about 25 minutes, adding the green beans for the final 5 minutes. Drain.
Place the salmon skin-side down in the casserole dish and top with the tarragon-orange mixture. Bake until the salmon is cooked through but slightly pink in the middle, about 25 minutes.
Divide the salmon among plates. Strain the oil from the casserole dish into the large bowl with the orange juice mixture; season with salt and pepper, then add the potatoes and green beans to the dressing, tossing well. Let stand until the potatoes have absorbed some of the dressing, about 3 minutes. Serve with the salmon.