Heat olive oil in a large dutch oven or stockpot over medium heat. Add onions, salt, and a pinch of sugar (optional). Cook, stirring frequently, for 8–10 minutes. Add broccoli, cauliflower, carrots, and celery. Cook, continuing to stir, about 5–8 minutes. Add garlic and cook about 1 minute longer.
Remove 1–2 cups of vegetables from pot (depending how smooth or textured you prefer your soup); set aside and reserve.
Add all other ingredients to pot besides lemon juice; bring to a boil. Reduce to a simmer, and cook, uncovered for 20 minutes.
Using an immersion blender, blend soup until smooth (alternatively, blend in batches in standard blender). Add reserved vegetables and simmer until heated through and tender. Lastly, stir in fresh lemon juice. Season with S&P and adjust according to preference.