Warm a medium pot over medium heat.
Add the olive oil and butter. When melted and shimmering, add the sliced leek and a pinch of salt. Cook 4–5 minutes until soft and fragrant, but not browned.
Stir in the garlic and cook 30 seconds.
Add the quartered radishes and diced potato. Season with a little more salt and pepper. Cook 2–3 minutes to coat the vegetables and build flavor.
Pour in the broth.
Bring to a gentle boil, then lower to a simmer. Cook 15–18 minutes, until the potatoes and radishes are very tender.
If using greens, stir them in during the last 2 minutes to wilt. This adds a soft green hue and extra nutrients without overpowering the radish flavor.
Blend the soup until smooth.
Use an immersion blender in the pot, or carefully transfer to a blender in batches. Blend until silky. Return to the pot if needed.
Stir in 2 tablespoons lemon juice and taste.
Adjust salt, pepper, and acidity. Add another splash of lemon if you like it brighter. For extra creaminess, whisk in yogurt or crème fraîche.
Ladle into warm bowls.
Swirl a spoonful of yogurt on top if using, and add a few slices of raw radish for crunch. Finish with chopped herbs, a drizzle of olive oil, and your floral garnish.
To garnish with flowers, gently separate petals or small blossoms. Scatter them lightly just before serving.
Use a light hand—the flowers are a highlight, not the whole show.