Method
Heat the butter and oil in a large pot. Add the diced onions and sauté until they are translucent and soft.
Add the flour to the onions. Stir constantly for 1-2 minutes until the flour smells nutty but hasn't changed color.
Toss in the diced zucchini. Stir for a few minutes to coat them with the flour and butter mixture.
Pour in 1 liter of water (or stock). Bring to a boil, then reduce heat and simmer until the zucchini pieces are completely soft.
Use an immersion blender to process the soup until it is perfectly smooth and velvety.
Pour in the milk while stirring. This will lighten the color and add a wonderful creaminess. If the soup is too thick, add a little more hot water.
Add the finely chopped dill and salt. Let it simmer for another 2 minutes, then turn off the heat.
In a small pan, melt a bit of butter with red pepper flakes and drizzle it over each bowl before serving.
Notes
After adding the milk, keep the heat low and avoid boiling the soup vigorously. Gentle simmering helps keep the soup smooth and prevents the milk from separating.