1 (12 fluid ounce) can or bottle Irish stout beer (such as Guinness®)
Gently pour half the lager beer into a tall beer glass. Place a large tablespoon, dome side up, an inch or so above the lager beer, with the tip of the spoon pointed slightly downhill. Slowly pour half the stout beer over the tablespoon, so the stout gently pours down the side of the glass in a thick trickle. Allow to stand a few seconds so 2 distinct layers of beer form.