This is just a short reminder to all C# developers defining and implementing interfaces with a single method:

What you are looking for is delegate

A delegate is a way to define the signature of a method. It can then be implemented by a Func, an Action or class method, whichever fits the best.

They even work nicely with dependency injection: a delegate is just a definition, not an implementation. If you want to preserve testability, that's taken care of: in tests you can just pass in a dummy implementation by way of a Func or Action; no mocking framework needed.

Here's an example

// Definition
public delegate void SendEmail(string recipient, string sender, string subject, string message);

// Registering with a DI system
services.AddTransient(_ => (recipient, sender, subject, message) => { /* Implementation */ });
// or
services.AddTransient(_ => StaticClassWithImplementation.StaticMethodImplementingDelegate);

// In testing
var systemUnderTest = new ClassThatNeedsToSendEmails((_, __, ___, ____) => { log.Info("Sent email"); });

Don't overcomplicate with interfaces and classes, what could simply be a delegate.