How can I avoid IEnumerable nested in another IEnumerable using lambda expressions

Question

I am refactoring some code that currently consists of a foreach inside another foreach to get a list of types. Later on on those types some action is performed.

At the moment I end with an IEnumerable inside another IEnumerable, I wanted to have the final list of types (e.g. IEnumerable or Type[]) to process.

How can I unwind my result, so that I don't have a nested IEnumerable (IEnumerable inside an IEnumerable)?

This is (simplified) my current code:

var types = assemblies.Select(asm => asm.GetExportedTypes()
                        .Where(t => typeof(IMyCustomValidator).IsAssignableFrom(t) && !t.IsInterface));

This results in types being of type IEnumerable<IEnumerable<Type>>.

I believe there should be a rather simple solution for this issue but I cannot get my head around it.