How to define a custom method with implementation in ReactiveMongoRepository?

Question

How to define a custom method (with implementation) in SpringData ReactiveMongoRepository?

This method is supposed to be a stateless combination of calls to other methods in MyCollectionRepository and there is no need to generate a SpringData query from its name.

interface MyCollectionRepository : ReactiveMongoRepository<MyCollection, String> {

    fun findSomething(param: Long): Flux<MyCollection> {
        return Flux.empty<MyCollection>()
    }
}

On application context initialization this exception is thrown:

org.springframework.data.mapping.PropertyReferenceException:
     No property findSomething found for type MyCollection!