Add one object in list nested entity in EF Core

Question

I create a repository layer in my app. I don't need all list EntityList. I want add in Foo.EntityList 1 object which equals entityId, and return foo in notImportant. How can I do this?

I am using EF Core 3.1 - I can't use Where() in Include().

How can I realize this contruction in EF Core 3.1?

 Foo.Include(c => c.Entity.Where(o => o.Id == entityId)).ThenInclude(o => o.EntityProperty) 

I thought to do it with Join() but I don't know how.

This is the method in my repo:

Task<NotIpmortantEntity> FetchNotImportantEntity(int year, Guid foosId, Guid entityId);
{
    var db = Datasource();
    var foo = db.Foo.Where(/???/)   
    var notImportant = NotIpmortantEntity.New(year, Foo);
}

Foo has these properties:

Foo:
|--Guid FoosId
|--IList<Entity> EntityList  

And Entity has these properties:

Entity:
|--Guid EntityId