Preheat oven to 350 degrees. Line a small baking sheet with foil.
Place the samon on the baking sheet and sprinkle with salt and pepper. Bake until cooked through, about 20 minutes. Use a fork to flake the salmon.
While the salmon cooks, heat a small skillet to a medium heat. Once the pan is hot, add 2 teaspoons oil. Add onion, celery, and garlic. Season with a ¼ tsp. salt. Sweat the vegetables (turn the heat down if they start to brown) until they're slightly softened, about 5 minutes. (You still want them to have a litlte bit of a bite to them.) Cool for a minute or two.
Add the egg, dijon, dill, mayo, lemon juice, and remaining ¼ teaspoon salt to a medium bowl. Whisk to combine. Add the flaked salmon, cooled veggies, panko, and all but 2 tsp. of the green onion to the wet ingredients. Use a fork to stir the mixture until combined. Form into four large or eight small patties.
Heat a large non-stick skillet or griddle to a medium heat. Brush with remaining olive oil. Add salmon patties, and cook until golden brown and crispy, about 3-4 minutes per side.
Serve with Tzatziki Sauce and garnish with remaining green` onion.
