Heat olive oil in a large pot over medium heat and add the diced onion. Cook for 6-8 minutes, stirring occasionally, until the onions are golden and caramelized.
Add the minced garlic and cook for another minute until fragrant.
Pour in the vegetable broth and bring to a boil, then add the asparagus, peas, and zucchini. Reduce heat and simmer for 10-12 minutes until vegetables are tender.
Stir in the spinach, thyme, salt, and pepper, cooking for 2-3 minutes until the spinach is wilted.
Remove from heat and stir in the fresh lemon juice. Taste and adjust seasoning as needed before serving.