Preheat oven to 350 degrees F. Grease an 8-inch round cake pan and lightly flour all sides. Set aside.
In a medium bowl, whisk together flour, brown sugar, salt and slivered almonds. Pour in about half the butter and mix. Keep adding more until the mixture forms a crumbly texture. You may not need to use all the butter, but you don't want extra flour kicking around the bottom of the bowl.
In the bowl of a stand mixer, fitted with the paddle attachment, beat together butter and sugar until light and fluffy, about 5 minutes.
Add in eggs and beat until combined.
Add in lemon zest and lemon juice and beat until well combined.
In a separate medium bowl, whisk together flour, baking powder and salt.
Add the flour mixture to the butter mixture and mix for about 20 seconds. Add in poppy seeds and sour cream. Beat until mixture is well combined.
Transfer the batter into the prepared 8-inch pan. Spread the batter evenly with an offset spatula.
Top with streusel.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Allow the coffee cake to cool in the pan for about 15 minutes. Run a knife around the outside edges to encourage it to release easily. Invert onto a plate, and then onto a cooling rack to finish cooling.
Recipe adapted from Bake from Scratch Magazine Vol 3 Issue 1