Preheat oven to 400°F.

Chop potatoes into one-inch cubes, toss with 2 tablespoons grapeseed oil and transfer to baking sheet or roasting pan. Roast in the oven for 15 minutes.

Remove and discard roots and green tops from leeks. Wash thoroughly under running water to remove grit. Cut white portions into large slices, toss with 1 tablespoon grapeseed oil and salt and add to the baking sheet with the potatoes. Roast potatoes and leeks together for an additional 10 minutes.

Transfer roasted potatoes and leeks to a food processor. Add garlic, mushroom broth, herbs, salt and pepper and blend until smooth. Reheat and serve.