BsonClassMap.RegisterClassMap<Pages>(cm =>
{
    cm.AutoMap();
    cm.MapIdMember(c => c.Id);
});

BsonClassMap.RegisterClassMap<PageContainer>(cm =>
{
    cm.AutoMap();
    cm.MapIdMember(c => c.Id);
});

var client = new MongoClient("mongodb://localhost:27017");
var database = client.GetDatabase("playground");

var pages = database.GetCollection<Pages>("pages");
var pageContainers = database.GetCollection<PageContainer>("pagecontainers");

var pagesWithContainers = await (from page in pages.AsQueryable().Where(x => x.TenantId == 1)
    join container in pageContainers.AsQueryable() on page.Id equals container.PageId into containers
    select new
    {
        page.Id,
        page.Title,
        Containers = containers
    }).ToListAsync();  

Console.ReadLine();

public class Pages
{
    public required ObjectId Id { get; init; }
    public required string Title { get; init; }
}

public class PageContainer
{
    public required ObjectId Id { get; init; }
    public required ObjectId PageId { get; init; }
    public required string Content { get; init; }
} 

Unhandled exception. MongoDB.Driver.Linq.ExpressionNotSupportedException: Expression not supported: playground.pagecontainers.Aggregate([]).Where(x => (x.TenantId == 1)) in playground.pages.Aggregate([]).Where(x => (x.TenantId == 1)).GroupJoin(playground.pagecontainers.Aggregate([]).Where(x => (x.TenantId == 1)), page => page.Id, container => container.PageId, (page, containers) => new <>f__AnonymousType0`3(Id = page.Id, Title = page.Title, Containers = containers)) because inner expression must be a MongoDB IQueryable against a collection.
   at MongoDB.Driver.Linq.Linq3Implementation.ExtensionMethods.ExpressionExtensions.GetCollectionInfo(Expression innerExpression, Expression containerExpression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToPipelineTranslators.GroupJoinMethodToPipelineTranslator.Translate(TranslationContext context, MethodCallExpression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToPipelineTranslators.ExpressionToPipelineTranslator.Translate(TranslationContext context, Expression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToExecutableQueryTranslators.ExpressionToExecutableQueryTranslator.Translate[TDocument,TOutput](MongoQueryProvider`1 provider, Expression expression, ExpressionTranslationOptions translationOptions)
   at MongoDB.Driver.Linq.Linq3Implementation.MongoQuery`2.ToCursorAsync(CancellationToken cancellationToken)
   at MongoDB.Driver.IAsyncCursorSourceExtensions.ToListAsync[TDocument](IAsyncCursorSource`1 source, CancellationToken cancellationToken)
   at Program.<Main>$(String[] args) in /Users/keremacer/Projects/Playground/Playground/Program.cs:line 25
   at Program.<Main>(String[] args)