Preheat oven to 425°F. Arrange potatoes, cut side down, on a large rimmed baking sheet, spacing evenly apart. Add 1 1/4 cups water (just enough to coat the baking sheet), and place in the oven. Bake for 25 to 30 minutes, until the potatoes are fork-tender and water has evaporated. (Note: if any water remains, blot it with a kitchen towel or carefully drain in the sink.)
While the potatoes bake, begin preparing salmon. Pat salmon dry with a paper towel, and brush 1 Tbsp. of the oil evenly over flesh and skin. In a small bowl, combine chili powder, garlic powder, and 1/2 tsp. salt. Sprinkle seasoning mixture evenly over salmon flesh, and let stand 5 to 10 minutes.
Push potatoes to one half of the baking sheet and toss with 1 1/2 Tbsp. oil, 1/2 tsp. salt, and 1/4 tsp. black pepper. Arrange salmon fillets in the center of the baking sheet, and transfer back to the oven. Bake for 5 minutes.
Add green beans to the open side of the pan, and toss with remaining 1/2 Tbsp. oil, 1/4 tsp. salt, and 1/4 tsp. black pepper. Place pan back in the oven and bake for 10 minutes. Turn on broiler to high; broil until salmon is golden, about 2 minutes.
During the final bake, prepare Garlic Herb Sauce by combining sour cream (or mayo), lemon juice, parsley, Dijon mustard, and garlic in a small bowl. Stir with a whisk, and season to taste with salt and pepper. Stir in 1 to 2 tsp. water if you prefer the sauce a little thinner.
Garnish salmon with lemon zest, and potatoes with additional chopped fresh parsley, if desired. Serve with Garlic Herb Sauce.