Preheat a 10-in. fry pan on medium heat until the rim is hot to the touch. Add the oil and butter and let the butter melt. Swirl the butter into the oil.
When the foam from the butter subsides, add the leeks. Sauté the leeks until tender and transparent, about 3 minutes.
In a 6.5- qt. sauce pot, heat the vegetable broth on medium-high heat. When broth reaches a gentle boil, add all other ingredients. Continue to heat on a gentle boil for 15 - 20 minutes.
Serve with crusty bread.