In a medium-sized pot, heat the avocado oil over medium heat. Add chopped yellow onion and cook for 3-4 minutes until translucent.
Add trimmed asparagus, minced garlic, sea salt, and black pepper. Cook for another 3-4 minutes.
Add vegetable stock and bring to gentle boil. Once boiling, reduce heat to a simmer and cook uncovered for 15 minutes.
Add sweet green peas and tarragon, simmer for another 2-3 minutes.
Remove from heat and cool for 5 minutes.
Use an emersion blender, or transfer contents of pot to a blender with a secure lid. Blend until soup is completely pureed and smooth. (Pureeing in batches may be helpful if you are using a smaller sized blender)
Return to pot and add lemon juice if using. Stir to combine and adjust sea salt and black pepper to taste.