Cook the onions in a couple of Tablespoons of olive oil over medium heat until translucent.
Add carrots and potatoes and tomatoes. Cover with water. Bring to a hard boil for 8-15 minutes, until potatoes are soft (will go faster the smaller the pieces are).
Transfer contents to a blender. Puree until consistency of thick cream. Hold back/ add more water/ stock as necessary to achieve desired consistency.
Can bring back to pot and hold at low heat or serve immediately.