This is a really easy way to fill in the blank spaces in your cutwork. (Please ignore the weird color combo - I had to place a contrasting piece of paper on my scanner so the stitch would show.)
To create this stitch, weave or wrap your bars. In the picture, you'll notice that these bars are wrapped - I was feeling lazy tonight. :) The filling stitch starts by coming up in the center hole of the remaining fabric threads at the corners of the bars. With your needle on the top side of the fabric, go diagonally across the open space to the corresponding remaining fabric grid. Go down through the center hole of that grid, and loop back to your starting hole, twisting your needle around the working thread twice - finish with your needle on top of the fabric. Go down through your starting hole, and needle under the wrapping to finish your cross. Come up again through the center hole of the fabric grid, go diagonally to the final unused fabric grid - either over or under the first pass, it's your choice. Go down in the center hole, and pass back along the path of your thread. Twist around your working thread, making sure to catch the "cross" point of your cross stitch. Finish twisting with your needle on top of the fabric, and go down through the center hole again.
It's much more difficult to explain than to do. If the picture doesn't help, let me know and I'll try to take pictures of the thread path as I do them. If you look at the larger image, you'll see I still have 3 more to do on this piece - it shouldn't take me more than 30-45 minutes to finish the piece (including bar wrapping).