Place yellow cake layer with crown removed onto plate or foil-covered cake board. Pipe tip 104 green buttercream ruffle "lettuce" along top edge of layer. Top with brownie layer. Outline and pipe in tip 7 yellow buttercream "cheese slice"; smooth with finger dipped in cornstarch or damp brush. Pipe tip 7 red buttercream lines in semi-circle for "tomato slice"; add tip 44 (smooth side out) edge of "slice". Pipe white tip 366 outlines for onion slice. Top with remaining yellow cake layer, dome side up. Brush rice cereal with piping gel; place on cake top.