Preheat the oven to 350°F.

Grease an 8- or 9-inch loaf pan with butter and dust with flour. Set aside.

To make the streusel topping, stir together 1/2 cup flour, 2 tablespoons sugar, 1 egg white, 2 teaspoons lemon zest and 1/4 of the salt just until mixture forms small lumps. Chill until ready to use.

In a large bowl, whisk together remaining 1 1/2 cups flour, baking powder, baking soda and remaining 1/2 teaspoon salt.

In a second large bowl, beat butter and remaining 1 cup sugar with an electric mixer for 1 minute.

Add remaining 2 egg whites one at a time, beating well after each addition.

Add remaining 4 teaspoons lemon zest, lemon juice and ginger and beat again until combined.

Add flour mixture in thirds, alternating with sour cream and beating well after each addition.

Transfer batter to prepared pan, scatter reserved streusel over the top and bake until cooked through, golden brown, and a toothpick inserted in the middle comes out clean, about 1 hour.

Set aside to let cool completely, then turn out of pan and serve.