Heat the olive oil in a large pot, then cook the onion for about 4 minutes until soft.
Stir in the garlic and thyme, cooking for another 30 seconds.
Add the potatoes, vegetable broth, Dijon mustard, salt, and black pepper.
Bring the soup to a boil, reduce the heat, and simmer for about 10 minutes.
Add the green beans and continue simmering for 10 to 12 minutes until both vegetables are tender.
Mash a few potato pieces against the side of the pot to thicken the broth slightly.
Stir in the lemon juice and parsley, then taste and serve.