Heat oil in a medium saucepan over medium-high heat. Add onion, garlic, 1/2 teaspoon salt and a pinch of pepper. Cook, stirring for 3 minutes. Add vegetable stock and bring to a boil.
Add peas and return to boil. Reduce heat and simmer, stirring occasionally until peas are tender and bright green, for 2 to 3 minutes.
Stir in spinach. Cover and cook, stirring occasionally, for another 2 to 3 minutes.
Working in 2 batches, puree pea mixture in a blender. Use water if required to achieve pouring consistency.
In a small bowl, mix coriander with lemon juice, olive oil, salt and pepper.