For the biscuits, preheat the oven to 425°F. Line two large baking sheets with parchment paper; set aside.

Sift flour, baking powder, baking soda and salt into a large bowl.

Add butter and use your fingers to work it into the flour mixture until it resembles coarse breadcrumbs.

Add buttermilk, chives and dill and stir together with a fork just until combined.

Turn dough out onto a floured surface and press into a disc.

Roll out dough until 3/4-inch thick.

Using a floured 2 1/2-inch biscuit cutter, cut out biscuits and transfer to prepared baking sheets, spacing them 1 to 2 inches apart.

Gather remaining pieces of dough, press them together gently and roll them out again until all of the dough has been used.

Brush egg and water mixture over the tops and sides of all of the biscuits and bake until cooked through and light golden brown, 12 to 14 minutes.

Meanwhile, for the creamy chive spread, put cream cheese, zest, 1 teaspoon of the dill, chives and pepper into a medium bowl and stir together until combined.

Cover and set aside at room temperature.

To serve, split warm biscuits in half and spread bottom halves with some of the creamy chive spread.

Layer salmon, tomatoes and onions over cream cheese, then garnish with remaining 2 tablespoons dill and top with remaining biscuit halves. Serve immediately.