Peel squash and grate through the large holes of a special squash slicer or standard 4-sided metal grater. Stop grating when you get to the seeds. Or halve the squashes and remove seeds before you start grating. You should end up with a mound of matchsticks, not pulp!
Sprinkle with the salt, mix, and let stand under a cover for 1 hour. Squeeze squash and set aside. (To keep summer squash or other summer vegetables from becoming mushy, rinse them in vinegar for a second before you start cooking them.)
Meantime make a light roux out of butter and flour. Add onion. When onion becomes golden brown, add the dill.
Put the roux in a large pot and mix well with stock or water. Add squash slaw. Cook for 10 minutes.
Add 1tablespoon vinegar or lemon juice and the sugar, and bring to a boil. Adjust sweetness or sourness to taste. Mix in sour cream and serve.