Combine the mayonnaise, yogurt, scallions, lemon juice, dill, and 1/4 teaspoon of the pepper in a small bowl. Mix well, cover, and set aside.

Preheat oven to 450˚F. Lightly spray a baking sheet with vegetable oil.

Heat 1 1/2 teaspoons of the oil in a large skillet over medium-high heat. Add the onion and celery and sauté about 3 minutes, until lightly softened. Add the parsley and remove from heat.

Place the salmon in a mixing bowl. Remove any bones or skin, and flake apart with a fork. Add the beaten egg and Dijon mustard and mix well. Stir in the sautéed onion and celery, then the breadcrumbs and remaining pepper. Shape into 8 patties about 2 1/2 inches wide.

Heat the remaining 1 1/2 teaspoons of olive oil in the skillet over medium heat. Arrange 4 of the patties in the pan and cook, undisturbed, for 2-3 minutes, until the underside is golden. Use a large spatula to transfer the patties to the prepared baking sheet, golden side down. Repeat with the remaing 4 patties. 

Bake until golden on top and hot all the way through, 15-20 minutes.

Serve with the creamy dill sauce and slices of lemon.