Heat the olive oil in a large saucepan over medium heat until shimmering. Add the chopped onion and cook, stirring occasionally, until it becomes translucent and soft, about 5 minutes, fragrant with a slight sizzle.
Add the minced garlic and cook for another 30 seconds, until fragrant and just beginning to brown lightly. Stir frequently to prevent burning.
Pour in the vegetable broth and bring to a gentle simmer. Once simmering, add the spinach leaves and mint leaves, stirring gently to submerge the greens.
Allow the mixture to simmer uncovered for about 10 minutes, until the spinach wilts completely and the mixture thickens slightly with a vibrant green color.
Remove the saucepan from heat and use an immersion blender to puree the soup until smooth, or transfer carefully in batches to a blender. Blend until silky and uniform, with no visible chunks.
Stir in the heavy cream for added richness, then season with salt and black pepper to taste. Mix well and adjust seasoning if needed.
Return the soup to low heat for a minute or two to heat through, then taste and adjust the seasoning as desired.
Serve the soup hot, garnished with a few fresh mint leaves or a drizzle of cream if desired.
