Activate Yahenni by sacrificing Cauldron Familiar, giving Yahenni indestructible until end of turn.
Activate Cauldron Familiar by sacrificing a Food, returning Cauldron Familiar to the battlefield from your graveyard.
Cauldron Familiar and Insidious Roots trigger.
Resolve the Insidious Roots trigger, creating a 0/1 green Plant creature token and a Food artifact token, and then putting a +1/+1 counter on each Plant you control.
Resolve the Cauldron Familiar trigger, causing each opponent to lose 1 life and you to gain 1 life.