Using a medium-sized saucepan, combine two cups of water and chopped potatoes --- bring to a boil over medium-high heat.
Adjust the heat to medium-low and cook the potatoes until they're slightly tender.
Combine the cream and flour until the flour has completely dissolved, then set aside.
Once the potatoes are soft, add the carrots, green beans, green onions, butter, salt, and pepper.
Continue simmering until the carrots and beans are slightly cooked.
Mix green peas with the rest of the summer vegetables into the pan.
Add the flour and cream mixture and stir to combine.
Bring summer soup to a quick boil --- taste and adjust seasoning, if needed.
Remove the pan from the heat and transfer it into bowls.
Serve and enjoy your Finnish summer vegetable soup!