How to Find a record in Entity Framework without a Primary Key

I've got an entity:

public class Account
{
    public int AccountId { get; set; }
    public string Mnemonic { get; set; }
    public decimal NetAssetValue { get; set; }
}

On this entity I have a primary key (AccountId) and an alternate unique index on the mnemonic.

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    // Account
    modelBuilder.Entity<Account>()
        .HasKey(a => a.AccountId);
    modelBuilder.Entity<Account>()
        .HasIndex(a => a.Mnemonic)
        .IsUnique();
}

When I store my test data in an XML file, I have no knowledge of the value assigned to the primary key, so I need to find this record by the Mnemonic if I want to use it. I know I can use LINQ:

var accountId = (from a in account
                where mnemonic = "Account1"
                select AccountId).First();

But will this use the index or iterate over the entire collection. I could have thousands of accounts and don't want do be executing a table scan each time I want to find an account when I'm loading from my external files.