throw (someone) in at the deep end

Translations
погрузить (кого-то) с головой в сложную ситуацию
When she joined the company, they threw her in at the deep end with a challenging project.
Когда она присоединилась к компании, её погрузили с головой в сложный проект.
Definitions
To put someone in a challenging or difficult situation without much preparation, support, or guidance, often to see how they cope or to encourage them to learn quickly.