Grease with shortening a large bundt pan (10 or 12-cup capacity), flour it, shake off excess, and reserve while making the batter.
In a large mixing bowl (stand mixer or with an electric hand mixer), beat butter for a few seconds until creamy.
Add sugar gradually while beating at medium/high speed for 3 minutes. It should be light-colored and fluffy.
Add eggs, one at a time, beating well after each addition.
Add vanilla extract, lemon zest, and mix to combine.
Add the sifted dry ingredients (flour, baking soda, and salt) in 3 additions, alternating with milk in 2 additions. You can sift the dry ingredients together in a different bowl before adding them or have them measured and sift them directly over the butter mixture.
Pour half of the batter into the prepared pan. Smooth top.
Add half the frangipane in small mounds and lightly swirl with a knife.
Cover with the rest of the batter, spreading evenly.
Repeat with the remaining almond cream, swirling lightly.
Bake for 50-55 minutes, or until a cake tester or toothpick in the middle comes out clean. All ovens and pans are different so start checking at 45 minutes. Mine was baked at 55.
Let cool for about 15 minutes, then carefully remove to a wire rack and let cool completely before glazing.
Pour the glaze evenly on the completely cooled cake, letting it drip down the sides. Scoop the drips and drizzle it again before it dries. The icing is thin, so it will drip easily.
Sprinkle with sliced almonds immediately, before it sets.
Keep covered at room temperature for a day or two and then refrigerate covered in plastic wrap or in an airtight container. You can freeze it for a month.