How to /summon an item riding a mob?

Can someone help me with it? I know there's a way to summon mobs, of course, also items, but can you stack them together so the item is on top? I found no way to do it, perhaps i just do something wrong...

The Passengers list tag will hold a list of entities that are riding the host entity:

/summon Zombie ~ ~1 ~ {Passengers:[{id:"Item",Item:{id:"minecraft:stone",Count:1b},PickupDelay:32767s}]}

Being a list, you can have multiple entities riding at the same depth (which is used for boats to carry multiple passengers):

/summon Zombie ~ ~1 ~ {Passengers:[{id:"Creeper"},{id:"Skeleton"}]}

You can also add the Passengers list to a passenger, such that they have their own passengers higher up:

/summon Boat ~ ~1 ~ {Passengers:[{id:"Creeper"},{id:"Skeleton",Passengers:[{id:"Zombie"}]}]}

Wood block riding on an Ocelot:

/summon minecraft:ocelot ~ ~1 ~ {Passengers:[{id:"Falling_block",Time:1,TileID:162,Data:16}]}

If you want the block to move around on its own use this:

/summon minecraft:ocelot ~ ~1 ~ {ActiveEffects:[{Id:14,Duration:2000000,Ambient:1,ShowParticles:0b}],Passengers:[{id:"Falling_block",Time:1,TileID:162,Data:16}]}

Enjoy and have fun!