Coarsely grate the zucchini using the largest holes on a box grater.
Place in a colander, toss with a little salt and set aside to drain while you make the roux.
Melt the butter in a large frying pan over medium heat. Add the onion and sauté until soft and translucent, 3 to 4 minutes.
Sprinkle the flour over the onion and stir to combine with the butter. Continue cooking, stirring often, until the flour develops a pale golden color.
Whisk in the milk and cook until the mixture thickens to a creamy paste.
Add the zucchini, combine well and continue cooking for 5 to 7 minutes, stirring often.
Once the liquid given off by the zucchini has combined with the roux to form a light sauce, stir in the sour cream and vinegar.
Add the dill, combine well and season to taste with salt and pepper.
Transfer to a serving dish and garnish with a little extra dill if desired.
Tips for Making This Recipe
Use the largest holes on a box grater to grate your zucchini (or summer squash)
Once grated, add the zucchini to a colander, toss with a little salt, and set aside for a few minutes to draw out some of the moisture.
Watch your roux carefully and don't let it get too brown - a pale golden color is perfect for this dish
Adjust the amount of vinegar and fresh dill to suit your taste. The quantities of vinegar and dill you use depend on your own personal taste. We think the amounts in the recipe are fairly conservative and always end up adding a little extra of both.