It also exists the delayUntil method for Mono:

Mono<Foo> fooMono = fooService.loadFoo();
fooMono.delayUntil(foo -> barService.loadBarForFoo(foo))

Or, using method reference:

Mono<Foo> fooMono = fooService.loadFoo();
fooMono.delayUntil(barService::loadBarForFoo)

Method reference: Mono.delayUntil