The Portuguese stem stitch is another of the knotted stitches. You start with a basic stem stitch and then wrap your thread twice around the middle before starting the next stitch. It looks like a heavy rope or chain and would make a nice variation on stem or outline stitches if you want a thicker line or one with more interesting detail. It, too, is from my One Hundred Embroidery Stitches booklet so I'll reproduce the drawings in addition to my photos. Sometimes it helps to see it both ways and this one was particularly hard to photograph.
So, working on a drawn line, begin as for a stem stitch (A in the above drawing).
Pull the thread through and pass the needle (from right to left) under the stitch you just made without entering the fabric (B).
Pass the needle again under the same stitch and below the first coil (C). Keep your thread above the needle.
Make another stem stitch (D), keeping your thread below the needle.
Again, pass the needle twice under the stitch and also under the previous stitch (E). I've skipped ahead a few stitches but you'll get the idea.
When you come to the end of your line, take the thread to the back in the same hole as the end of the previous stitch.
And here's what it looks like when complete (F):
Slightly tricky at first but you'll pick it up quickly!