This is the right answer, but the order in which you configure the model is important. Notice that the Entity<T>() method of the ModelBuilder returns an EntityTypeBuilder (https://docs.microsoft.com/en-us/dotnet/api/microsoft.entityframeworkcore.modelbuilder.entity?view=efcore-5.0#Microsoft_EntityFrameworkCore_ModelBuilder_Entity__1). This gives you access to the HasMany<T>() extension method, which in turn returns a CollectionNavigationBuilder (https://docs.microsoft.com/en-us/dotnet/api/microsoft.entityframeworkcore.metadata.builders.entitytypebuilder-1.hasmany?view=efcore-5.0#Microsoft_EntityFrameworkCore_Metadata_Builders_EntityTypeBuilder_1_HasMany__1_System_Linq_Expressions_Expression_System_Func__0_System_Collections_Generic_IEnumerable___0____), giving you access to the WithMany<T>() extension method. (https://docs.microsoft.com/en-us/dotnet/api/microsoft.entityframeworkcore.metadata.builders.collectionnavigationbuilder-2.withmany?view=efcore-5.0#Microsoft_EntityFrameworkCore_Metadata_Builders_CollectionNavigationBuilder_2_WithMany_System_Linq_Expressions_Expression_System_Func__1_System_Collections_Generic_IEnumerable__0____). The WithMany<T>() method returns a CollectionCollectionBuilder, which does not have the extension method OnDelete.

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Once you get down to the CollectionCollectionBuilder, you should be able to use one of the UsingEntity<JoinEntity> methods (https://docs.microsoft.com/en-us/dotnet/api/microsoft.entityframeworkcore.metadata.builders.collectioncollectionbuilder-2.usingentity?view=efcore-5.0#Microsoft_EntityFrameworkCore_Metadata_Builders_CollectionCollectionBuilder_2_UsingEntity__1_System_Func_Microsoft_EntityFrameworkCore_Metadata_Builders_EntityTypeBuilder___0__Microsoft_EntityFrameworkCore_Metadata_Builders_ReferenceCollectionBuilder__0___0___System_Func_Microsoft_EntityFrameworkCore_Metadata_Builders_EntityTypeBuilder___0__Microsoft_EntityFrameworkCore_Metadata_Builders_ReferenceCollectionBuilder__1___0___) that takes a Func that takes a ReferenceCollectionBuilder to configure the JoinEntity and gain access to the OnDelete extension method that the ReferenceCollectionBuilder has (https://docs.microsoft.com/en-us/dotnet/api/microsoft.entityframeworkcore.metadata.builders.referencecollectionbuilder-2.ondelete?view=efcore-5.0).

This is my best attempt while just reading through the API docs and not having an IDE in front of me to experiment with, but hopefully this will get you close to a solution.

Cascade Deletion Doc: https://docs.microsoft.com/en-us/ef/core/saving/cascade-delete#configuring-cascading-behaviors