Creating Mock reference and setup methods for complex operations using MOQ

I have two methods , OpenCertificateStore and FindCertificateBySubjectName and implemented them as following:

 public void OpenCertificateStore()
        {
            if (_certificateStore == default(X509Store))
                _certificateStore = new X509Store(StoreLocation.CurrentUser);

            _certificateStore.Open(OpenFlags.ReadOnly | OpenFlags.IncludeArchived);
        }

        public X509Certificate2Collection FindCertificateBySubjectName(string certificateSubjectName)
        {
            X509Certificate2Collection certificates = new X509Certificate2Collection();
            if (_certificateStore != default(X509Store))
            {
                certificates = _certificateStore.Certificates.Find(X509FindType.FindBySubjectName, certificateSubjectName, true);
            }

            return certificates;
        }

I have my unit test as below:

[TestClass]
    public class MyHealthTests
    {
        private Mock<Logger> _logger;
        private Mock<MYCertificateManager> _certManager;

        [TestInitialize]
        public void Initialize()
        {
             _logger = new Mock<Logger>();
             _certManager = new Mock<MYCertificateManager>();
        }

        [TestMethod]
        public void PassName_FindCertiFicatebyName_ShouldReturnValid()
        {


            MyCertificateHelper myCertHelper = new MyCertificateHelper(_logger.Object,_certManager.Object);

            myCertHelper.OpenCertificateStore();
            var certNameCollection = myCertHelper.FindCertificateBySubjectName("Valid Cert Name");
            Assert.IsNotNull(certNameCollection);
            Assert.IsTrue(certNameCollection.Count > 0);
        }
    }

Which works fine , but it would be lot better if I can find a way to mock myCertHelper.

If I do mok them , it returns null as it's not querying actual certificate store.