In a large pot, sauté onion in oil until soft and translucent, about 5-7 minutes. Add squash, potatoes, and vegetable bouillon. Cover with water and bring to a boil. Reduce heat to simmer until vegetables are soft (about 20-30 minutes).
After vegetables are soft, blend them in the water using an immersion blender. Once the soup has a creamy consistency (add more water to your desired consistency), add salt to taste, trying not to over-salt. This may take a few pinches to get it just right.
Stir in brown sugar. Mix in 1 tablespoon balsamic vinegar, taste, and add more as desired. The final soup will have underlying notes of balsamic vinegar, however it should not overpower the other flavors. Serve topped generously with fresh dill.
